Added overload for `http::OK()` that takes ContentType.
Added overload for `http::OK()` that takes ContentType.

Needed this for subsequent review.

Review: https://reviews.apache.org/r/48011

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