allow json_encoder kwarg when passing exc

[tysonholub]

With the json_encoder change in https://github.com/jamielennox/requests-mock/pull/198/commits/f70ba9277f580ecb6dec1daee3487b5b45772f3e, our tests now need to set our custom encoder in a requests_mock fixture. This causes the following to fail as the json_encoder kwarg is passed into the _MatcherResponse

self = <requests_mock.response._MatcherResponse object at 0x7f3c068f2b60>, kwargs = {'json_encoder': <class 'RestResponse.objects.RestEncoder'>}

    def __init__(self, **kwargs):
        self._exc = kwargs.pop('exc', None)
    
        # If the user is asking for an exception to be thrown then prevent them
        # specifying any sort of body or status response as it won't be used.
        # This may be protecting the user too much but can be removed later.
        if self._exc and kwargs:
>           raise TypeError('Cannot provide other arguments with exc.')
E           TypeError: Cannot provide other arguments with exc.