Failing test for #270.
Merge proxy_headers in HTTP.
More informative comment about proxy headers.
Stop applying special headers to proxies.
Apply proxy headers to a copy of the dict.
Revert "Stop applying special headers to proxies."

This reverts commit 09d03c9a75ba81d4469920b2d91df6d3ab4d7499.
Better comment here. Don't set proxy_headers twice.
Remove unexpected non-ascii byte.
Merge pull request #271 from Lukasa/proxy_headers

Resolves #270.