Fix "generic" specification

[juliens]

This PR fixes the h2spec "generic" part.

44 tests, 44 passed, 0 skipped, 0 failed

details

• Fix ping frame that needs an ack ping frame in response.
• Fix ignoring unkown frame.
• Fix partial header body with end of the body in a continuation frame.

For the partial header handling, this is a implementation proposal, things can be done differently, happy to talk about it.

[dgrr]

Looks very good. Thanks for the PR!