Fix tests on jruby 9000.
Fix tests on jruby 9000.

For some reason it appears that Jruby 9000 does not implement the
HTTP_PROXY environment variables introduced in Ruby 2.0 Net::HTTP. While
this is probably a bug, let's not worry about it for now.

crccw/rest-client,maxprig/rest-client,ab/rest-client,acrogenesis-lab/rest-client,antonklava/rest-client,rayzhng/rest-client,skv-headless/rest-client,JIFFinc/rest-client,keir-rex/rest-client,guoyu07/rest-client,landylee1989/rest-client,rest-client/rest-client,voxik/rest-client,korczis/rest-client,jonkgrimes/rest-client