Fixed issues with the Docker fetcher when using a proxy.
Fixed issues with the Docker fetcher when using a proxy.

When behing a proxy, 'curl' uses HTTP CONNECT tunneling to access HTTPS.
This lead to problems with our HTTP parser because the response of a
'CONNECT' doesn't have neither headers nor a body.

Review: https://reviews.apache.org/r/55621/

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