Forcing path_url to be of type str in Python2

This is important when posting files/binary data in Python2.
Since urlsplit will produce unicode parts, the path_url will also be unicode, and thus the sent body will be converted to unicode during concatenation by urllib, producing a UnicodeDecodeError when sending binary data.
Forcing the path_url to be a native string in Python2 resolves this.

No test cases yet.