Handle http_proxy settings from env with Ruby 1.9.X

Ruby 1.9.3 does not recognize the http_proxy settings in the way 2.X does.
http://ruby-doc.org/stdlib-1.9.3/libdoc/net/http/rdoc/Net/HTTP.html (search fpr http_proxy)
In order to ensure a consistent behavior with all ruby versions, the recognition of the http_proxy env variable for ruby < 2.0 was added.
Tests were added but only executed on Ruby 1.9.3. Would be great to test with different ruby versions.