Added unit test for http::Headers abstraction.
Added unit test for http::Headers abstraction.

Review: https://reviews.apache.org/r/56144/

chhsia0/mesos,chhsia0/mesos,zmalik/mesos,Gilbert88/mesos,jpeach/mesos,jpeach/mesos,chhsia0/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,jpeach/mesos,verizonlabs/mesos,chhsia0/mesos,abudnik/mesos,zmalik/mesos,reneploetz/mesos,dforsyth/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,kaysoky/mesos,andschwa/mesos,verizonlabs/mesos,jpeach/mesos,reneploetz/mesos,neilconway/mesos,zmalik/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,abudnik/mesos,andschwa/mesos,craimbert/mesos,neilconway/mesos,jpeach/mesos,asamerh4/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,dforsyth/mesos,verizonlabs/mesos,abudnik/mesos,abudnik/mesos,zmalik/mesos,abudnik/mesos,reneploetz/mesos,Gilbert88/mesos,andschwa/mesos,craimbert/mesos,dforsyth/mesos,dforsyth/mesos,yuquanshan/customizedMesos,gsantovena/mesos,abudnik/mesos,neilconway/mesos,chhsia0/mesos,dforsyth/mesos,kaysoky/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,craimbert/mesos,asamerh4/mesos,reneploetz/mesos,chhsia0/mesos,gsantovena/mesos,gsantovena/mesos,gsantovena/mesos,asamerh4/mesos,gsantovena/mesos,kaysoky/mesos,neilconway/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,neilconway/mesos,reneploetz/mesos,neilconway/mesos,zmalik/mesos,kaysoky/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,andschwa/mesos,jpeach/mesos,verizonlabs/mesos,yuquanshan/customizedMesos,neilconway/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,Gilbert88/mesos,Gilbert88/mesos,asamerh4/mesos,asamerh4/mesos,craimbert/mesos,craimbert/mesos,reneploetz/mesos,kaysoky/mesos,andschwa/mesos,craimbert/mesos,asamerh4/mesos,dforsyth/mesos,abudnik/mesos,gsantovena/mesos,reneploetz/mesos,zmalik/mesos,asamerh4/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,andschwa/mesos,gsantovena/mesos,andschwa/mesos,Gilbert88/mesos,kaysoky/mesos