Fix duplicate chain::doFilter.

Otherwise you get the following exception: https://gist.github.com/alesj/7674947
(this is a result of this test: https://github.com/GoogleCloudPlatform/appengine-tck/blob/master/tests/appengine-tck-channel/src/test/java/com/google/appengine/tck/channel/ChannelTest.java)