[issue] Resolving issue 2530 with non-ASCII characters in query string

[john-bodley]

This PR resolves issue #2530 related to having non-ascii characters in an explore query.

The actual fix was proposed by @xrmx here which remedies the problem in Python2 given that str(...) uses the ASCII codec, i.e.,

>>> str(u'Andalucía')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\xed' in position 7: ordinal not in range(128)

The fix is to use six.text_type(...) which calls unicode(...) in Python 2 and str(...) in Python 3.

>>> unicode(u'Andalucía')
u'Andaluc\xeda'

@xrmx I don't fully understand your comment,

but the code is still broken with non-ascii chars on python2
as the problem seems to be specific to Python 2 and using six.text_type(...) remedies the problem.

[xrmx]

@john-bodley i meant that there were other places where string were assumed to be ascii

[xrmx]

I'd prefer a commit title that gives some hints on what it is doing and a reference to the bug in the message but no big deal.

[john-bodley]

@xrmx I've updated the commit title. I agree with your comment where other strings were assumed to be ASCII. It seems there's a number of places which should apply similar logic.