'If the headers passed to L{Agent.request} includes a value for the
I{Host} header, that value takes precedence over the one which would
otherwise be automatically provided.'

def test_hostOverride(self):

    headers = http_headers.Headers({'foo': ['bar'], 'host': ['quux']})
    self.agent._getEndpoint = (lambda *args: self)
    self.agent.request('GET', 'http://example.com/foo?bar', headers)
    (req, res) = self.protocol.requests.pop()
    self.assertEqual(req.headers.getRawHeaders('host'), ['quux'])
