'Each header passed to the I{start_response} callable MUST hold a key
and a value, and ONLY a key and a value.'

def test_headersShouldEachHaveKeyAndValue(self):

    def application(environ, startResponse):
        startResponse('200    OK', [('too', 'many', 'cooks')])
        return iter(())
    (request, result) = self.prepareRequest(application)
    request.requestReceived()
    def checkMessage(error):
        self.assertEqual("header    must    be    a    (str,    str)    tuple,    not    ('too',    'many',    'cooks')", str(error))
    return self.assertFailure(result, TypeError).addCallback(checkMessage)
