Fix APIClient.get() when path contains unicode arguments

[diox]

If the URL contains unicode arguments, APIClient and APIRequestFactory .get() cause an exception to be raised when the request query dict is accessed, because we end up with an unicode QUERY_STRING and that's not what django expects.

Without this patch (which is adapted from django), the test I've added fails with:

tests/test_testing.py:252: in test_request_factory_url_arguments_with_unicode
    self.assertEqual(dict(request.GET), {'demo': [u'testé']})
../../.virtualenvs/drf-testing/local/lib/python2.7/site-packages/django/utils/functional.py:35: in __get__
    res = instance.__dict__[self.name] = self.func(instance)
../../.virtualenvs/drf-testing/local/lib/python2.7/site-packages/django/core/handlers/wsgi.py:124: in GET
    return http.QueryDict(raw_query_string, encoding=self._encoding)
../../.virtualenvs/drf-testing/local/lib/python2.7/site-packages/django/http/request.py:398: in __init__
    value = value.decode(encoding)
../../.virtualenvs/drf-testing/lib/python2.7/encodings/utf_8.py:16: in decode
    return codecs.utf_8_decode(input, errors, True)
E   UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in position 4: ordinal not in range(128)

[tomchristie]

This is great.
Thanks!