'Assert that a request with the given value in its I{X-Forwarded-For}
header is logged by L{proxiedLogFormatter} the same way it would have
been logged by L{combinedLogFormatter} but with 172.16.1.2 as the
client address instead of the normal value.
@param header: An I{X-Forwarded-For} header with left-most address of
172.16.1.2.'

def _xforwardedforTest(self, header):

    reactor = Clock()
    reactor.advance(1234567890)
    timestamp = http.datetimeToLogString(reactor.seconds())
    request = DummyRequestForLogTest(http.HTTPFactory(reactor=reactor))
    expected = http.combinedLogFormatter(timestamp, request).replace(u'1.2.3.4', u'172.16.1.2')
    request.requestHeaders.setRawHeaders('x-forwarded-for', [header])
    line = http.proxiedLogFormatter(timestamp, request)
    self.assertEqual(expected, line)
