Add ENABLE_PUSH flag in the Upgrade HTTP2-Settings header

[hkwi]

Push flag required for the case the initial upgrade request triggered server push.

[hkwi]

I'll look into.

[hkwi]

One travis-ci job is failing unexpectedly with empty raw log. Can I ask help?

[Lukasa]

I've restarted that build. =)

[hkwi]

I added http11 h2c upgrading test by inheriting TestServerPush which is for http20 h2, and test passed.

[hkwi]

I agree that HTTP11Connection.getpushes is a little bit hacky. This was added to meet the strict tests of test_promise_before_headers or test_capture_all_promises. If we don't need to satisfy these tests for http11 upgrade, implementation would be more relaxed style. I'll create another PR for that version later. BTW, the response will be stored in self._response for later self.get_response call, and would not lost.

[Lukasa]

I think those tests don't need to work for upgrade, but we should write a test that does work as expected (e.g. you get a HTTP/2 response and then look for pushes on that stream)