Pass strict to urllib3.

This change passes `strict=True` to urllib3, and thus to httplib.

As per the documentation:

> the optional parameter strict (which defaults to a false value) causes BadStatusLine to be raised if the status line can’t be parsed as a valid HTTP/1.0 or 1.1 status line.

Merging this would mean that requests is now actively limiting itself to HTTP/1.0+, excluding HTTP/0.9 (who the hell cares?). This is *already* requests behaviour on Python 3, it simply brings it into consistency in Python 2.

Note that, despite the fact that requests already sort of does this, this change is arguably breaking.