res.send(undefined) returns a 204
res.send(undefined) returns a 204

closes #600

Signed-off-by: Tj Holowaychuk <546b05909706652891a87f7bfe385ae147f61f91@vision-media.ca>

luxe-eng/valet-express.express,TestingCI/express,smcmurray/express,junxiong/express,za-creature/express,smcmurray/express,caiocutrim/express,pshomov/express,hanweifish/express,nodejs-camp/booklog-archer,hacksparrow/express,abdulhannanali/express,buildsample/express,youprofit/express,ZackBotkin/express,hl198181/express,michaeljunlove/express,bercanozcan/express,abdulhannanali/express,KodkodGates/express,abhi3/express,simudream/express,frootloops/express,r14r/fork_express_master,Ju2ender/express,nodejs-camp/booklog-polo,bjrmatos/express,leiqiang75/express,yinhe007/express,youprofit/express,soyuka/express,llwanghong/express,Jonekee/express,xtidt/express,za-creature/express,blakeembrey/express,1verson/express,MoraLin/mora-test,kylezhang/express,r14r/fork_express_master,pstrike/express,FernAbby/express,samuelcastro/express,1verson/express,vidyar/express,grigorkh/express,nodejs-camp/booklog-homa,Faiz7412/express,gretac/express,ci-test2/express,dongseop/express,codeforgeek/express,nodejs-camp/booklog-chle,JulyZhu/express,blairzhang/express,micha149/express,rgrove/express,ericlu88/express,qitianchan/express,aifeld/express,smcmurray/express,nathan818fr/express,PeterWangPo/express,WangCao/express,dreamingblackcat/express,emvici/emvici-reqres-tify,RanadeepPolavarapu/express,dotnetCarpenter/express,igarciaes/express,ericlu88/express,featurist/express,jinghuizhai/express,WangCao/express,shanawarkhan/express,topaxi/express,gnowoel/express,chuym/express,btleffler/express,angelsanz/express,za-creature/express,nodejs-camp/booklog-ben,featurist/express,flomotlik/express,sujianping/express,heicx/express,blakeembrey/express,CrazySherman/express,nodejs-camp/booklog-leo,demohi/express,Ju2ender/express,danielrohers/express,bercanozcan/express,XueQian/express,luxe-eng/valet-express.express,Ju2ender/express,hl198181/express,sibura/express,michaeljunlove/express,Lexine/express,qitianchan/express,saisai/express,sujianping/express,Flip120/express,saisai/express,tianhengzhou/express,kubrickology/express,nbeydoon/express,kubrickology/express,hacksparrow/express,corporateanon/express,simudream/express,fakedarren/express,hanweifish/express,Flip120/express,andriyl/express,friederbluemle/express,mauricionr/express,coreypnorris/express,bercanozcan/express,TestingCI/express,nathan818fr/express,jenalgit/express,philxia/ex-tst-framework,ahdinosaur/express,mimaidms/express,swstack/express,lmtdit/express,jenalgit/express,1amageek/express,srichatala/express,fineday32/express,YR/express,stewartml/express,devgeniem/express,nodejs-camp/booklog-jason,brianloveswords/express,tyler-johnson/express,strongloop/express,dieuly/express,vidyar/express,tyler-johnson/express,kunalpict/express,Matrixbirds/express,swstack/express,cflynn07/express,micha149/express,buildsample/express,Talkyunyun/express,juandav/express,andriyl/express,dafabulousteach/express,Faiz7412/express,fuse-mars/express,jinghuizhai/express,RanadeepPolavarapu/express,stevemao/express,elkingtonmcb/express,r14r-work/fork_express_master,fuse-mars/express,angelsanz/express,methuz/express,gdeividas/express,wtony/express,nodejs-camp/booklog-Zak,HXYNODE/express,supasate/express,michaeljunlove/express,juanpastas/express,simplyianm/express,expressjs/express,rgrove/express,fanzetao/express,awronka/express,tjmehta/express,abhi3/express,coreypnorris/express,nodejs-camp/booklog-Zak,adjohnson916/express,jimmykuo/express,kawanet/express,nodejs-camp/booklog-jollen,stewartml/express,Eric-Vandenberg/express,kunalpict/express,mcanthony/express,weilixia/express,chrisinajar/express,grigorkh/express,weilixia/express,CrazySherman/express,Talkyunyun/express,v10258/express,featurist/express,Eric-Vandenberg/express,drzorm/express,dgofman/express,sarvex/express,TestingCI/express,evanlucas/express,lmtdit/express,KodkodGates/express,swarajgiri/express,fineday32/express,Jonekee/express,cflynn07/express,auth0/express,Haru1990/express,PUSEN/express,Riokai/express,btleffler/express,nodejs-camp/booklog-ausir,nodejs-camp/booklog-ansaga,MoraLin/mora-test,stoskov/express,sarvex/express,coreypnorris/express,mikewesthad/express,dongseop/express,listepo/express,dgofman/express,corporateanon/express,kawanet/express,llwanghong/express,Ivanwangcy/express,dieuly/express,WebStyle/express,mauricionr/express,pshomov/express,wesleytodd/express,nodejs-camp/booklog-tonebeta,dotnetCarpenter/express,PUSEN/express,jenia0jenia/express,nodejs-camp/booklog-polo,srichatala/express,elkingtonmcb/express,dongseop/express,abdulhannanali/express,VegeChou/express,1amageek/express,microlv/express,qitianchan/express,nodejs-camp/booklog-tonebeta,RickyDan/express,flomotlik/express,pathirana/express,ifraixedes/express,udhayam/express,BaggersIO/express,nodejs-camp/booklog-polo,XueQian/express,lian774739140/express,juanpastas/express,devgeniem/express,svnh/express,blakeembrey/express,nodejs-camp/booklog-linkchen,fakedarren/express,andrejewski/express,stevemao/express,igarciaes/express,simplyianm/express,VegeChou/express,philxia/ex-tst-framework,friederbluemle/express,nodejs-camp/booklog-s890506,nodejs-camp/booklog-tonebeta-update,qq645381995/express,nodejs-camp/booklog-chle,tianhengzhou/express,drzorm/express,thepont/express,blairzhang/express,benshiue/booklog-ben,ci-test2/express,nodejs-camp/booklog-tonebeta-update,grigorkh/express,hightechweb/express,Riokai/express,KodkodGates/express,ahdinosaur/express,robomc/express,kentcdodds/express,LestaD/express,strongloop/express,kunalpict/express,nodejs-camp/booklog-jollen,LestaD/express,nodejs-camp/booklog-jason,pstrike/express,mikewesthad/express,FernAbby/express,wesleytodd/express,r14r-work/fork_express_master,svnh/express,swarajgiri/express,xiaofuzi/express,Kevnz/express,maraoz/express,jenia0jenia/express,gnowoel/express,isaacs/express,ci-test2/express,Eric-Vandenberg/express,simudream/express,dongooo/express,mimaidms/express,qq645381995/express,JulyZhu/express,listepo/express,HXYNODE/express,demohi/express,lmtdit/express,sarvex/express,benshiue/booklog-ben,mcanthony/express,abhi3/express,pathirana/express,v10258/express,evilboss/express,vinodhalaharvi/express,dsupera13/express,stoskov/express,kylezhang/express,xiaofuzi/express,vinodhalaharvi/express,nodejs-camp/booklog-mora,v10258/express,Ivanwangcy/express,lian774739140/express,expressjs/express,soyuka/express,stewartml/express,aifeld/express,blainsmith/express,nodejs-camp/booklog-s890506,WebStyle/express,danielrohers/express,blairzhang/express,samuelcastro/express,rupanjanbaidya/express,hightechweb/express,wtony/express,jenia0jenia/express,nodejs-camp/booklog-archer,gdeividas/express,gnowoel/express,tjmehta/express,junxiong/express,nodejs-camp/booklog-leo,stoskov/express,devgeniem/express,YR/express,fanzetao/express,wtony/express,Flip120/express,XueQian/express,nbeydoon/express,dreamingblackcat/express,jenalgit/express,hanqingnan/express,Kevnz/express,dotnetCarpenter/express,Lexine/express,chrisinajar/express,blainsmith/express,hpcslag/express,jimmykuo/express,rupanjanbaidya/express,nodejs-camp/booklog-chenyou,juandav/express,swarajgiri/express,isaacs/express,gretac/express,topaxi/express,svnh/express,methuz/express,kylezhang/express,qq645381995/express,bjrmatos/express,shanawarkhan/express,maraoz/express,hpcslag/express,evilboss/express,yinhe007/express,philxia/ex-tst-framework,hdngr/express,andrejewski/express,junxiong/express,ZackBotkin/express,dafabulousteach/express,supasate/express,1amageek/express,RickyDan/express,vinodhalaharvi/express,nodejs-camp/booklog-ausir,baldurh/express,microlv/express,lemelon/express,vnglst/express,xiaofuzi/express,neomadara/express,sibura/express,youprofit/express,kentcdodds/express,vnglst/express,listepo/express,friederbluemle/express,hpcslag/express,nodejs-camp/booklog-linkchen,caiocutrim/express,awronka/express,hdngr/express,stevemao/express,vidyar/express,corporateanon/express,ZackBotkin/express,udhayam/express,chrisinajar/express,lemelon/express,BaggersIO/express,countdracular/express,jymsy/express,fakedarren/express,hanqingnan/express,adjohnson916/express,nodejs-camp/booklog-homa,nbeydoon/express,countdracular/express,philxia/ex-tst-framework,dafabulousteach/express,jymsy/express,Haru1990/express,auth0/express,dsupera13/express,heicx/express,hdngr/express,LestaD/express,topaxi/express,evilboss/express,vnglst/express,PeterWangPo/express,frootloops/express,nodejs-camp/booklog-mora,codeforgeek/express,leiqiang75/express,WangCao/express,robomc/express,nodejs-camp/booklog-ben,luxe-eng/valet-express.express,xtidt/express,Riokai/express,nodejs-camp/booklog-chenyou,chuym/express,supasate/express,christopheranderson/express,Matrixbirds/express,ralic/express,thepont/express,dreamingblackcat/express,angelsanz/express,christopheranderson/express,ifraixedes/express,buildsample/express,nodejs-camp/booklog-ansaga,dongooo/express,baldurh/express,jinghuizhai/express,BaggersIO/express,Faiz7412/express,evanlucas/express,ralic/express,maraoz/express,neomadara/express