'Each header key passed to the I{start_response} callable MUST be at
native string in Python 2 and Python 3.'

def test_headerKeyMustBeNativeString(self):

    key = ('key' if _PY3 else u'key')
    def application(environ, startResponse):
        startResponse('200    OK', [(key, 'value')])
        return iter(())
    (request, result) = self.prepareRequest(application)
    request.requestReceived()
    def checkMessage(error):
        self.assertEqual(("header    must    be    (str,    str)    tuple,    not    (%r,    'value')" % (key,)), str(error))
    return self.assertFailure(result, TypeError).addCallback(checkMessage)
