googlev3 geocoder: Only encode given address as UTF-8 when running on Python 2

str(bytes()) == repr(bytes()) in Python 3, if not given an encoding
as well, making the whole exercise pointless.
The unicode string passes through just fine on the other hand.
Merge pull request #19 from akanouras/akanouras/py3k

googlev3 geocoder: Only encode given address as UTF-8 when running on Py...