Extracts the header containing the JSON web token from the given
        request.

def get_header(self, request):
        """
        Extracts the header containing the JSON web token from the given
        request.
        """
        header = request.META.get('HTTP_AUTHORIZATION')

        if isinstance(header, str):
            # Work around django test client oddness
            header = header.encode(HTTP_HEADER_ENCODING)

        return header