fixed serving default `index.html`
fixed serving default `index.html`

See [net/http](http://golang.org/src/pkg/net/http/fs.go#L314)
trigrass2/martini,franciscocpg/martini,sandyway/martini,kalashnikov/martini,imjerrybao/martini,w4ngyi/martini,beatrichartz/martini,sendgrid/martini,huamichaelchen/martini,sygool/martini,alskjstl/martini,SmarkSeven/martini,mikeqian/martini,jjz/martini,CestusMagnus/martini,zj8487/martini,aperera/martini,mtojek/martini,basakakoel/martini,go-martini/martini,wxdublin/martini,ansiz/martini,wddqing/martini,cjhubert/martini,Affix/martini,folpindo/martini,treetopllc/martini,OscaRoa/martini,anilanar/martini,jeff235255/martini,Berkaroad/martini,mingderwang/martini,beni55/martini,cocotyty/martini,gcnonato/martini,williamzhang2013/martini,coffeehc/martini,lcwxz1989/martini,b54898533/martini,soheilbm/martini,fc-thrisp-hurrata-dlm-graveyard/flog,kingland/martini,mirrr/martini