Generate 'Host' header with proxies.
Don't spuriously set the 'Host' header.

Also some docstring changes.
Merge pull request #146 from Lukasa/host_header

Generate 'Host' header with proxies.
Style.
Fixed proxy test (#146).
CHANGES.
Fixed tests for py3.
Merge branch 'master' of ssh://github.com/t-8ch/urllib3