Test that connection meta is updated correctly

[edsko]

On the client, whenever we make a new request, and process the response headers, we call updateConnectionMeta. One important function of this is to update our knowledge of the peer's supposed compression algorithms: only after the first response do we know what compression algorithms they support. We should have a test that verifies that after the first message, all future responses are compressed.

[edsko]

Since we actually had a regression here, increasing priority to medium. It's easy to miss in tests, since compression negotiation (including no compression) is completely transparent.