Don't follow HTTP_PROXY when talking to localhost test server

Unlike urllib2, requests honours HTTP_PROXY by default.  This meant
test_uri_length_limit's test queries to http://localhost:$port/ would
hit the wrong localhost when $HTTP_PROXY was another host - resulting in a
meaningless test scenario and apparent failures with HTTP status codes
other than the expected REQUEST_URI_TOO_LARGE (414).

This change forces requests to go direct for those test queries.

Closes-Bug: #1329614
Change-Id: Ida3424ca9a750bd2d3f2e81dc7dfacc2a334adf1
Merge "Don't follow HTTP_PROXY when talking to localhost test server"