Holy grail: 'unquote_plus' w/consistent behavior

* unquote -> unquote_plus. Rationale: if a literal '+' is encountered in
  a URL, it must be a space, because encodeURIComponent escapes '+' to
  '%2B'.
* Add version of 'unquote_plus' in compat.py that behaves consistently
  across Python 2 and 3.

Change-Id: Id1767b66b38ca110b2bd493eedaf68f2b1988c39
Merge "Holy grail: 'unquote_plus' w/consistent behavior"