http1 parser: reset before bailing out upgrade requests

when a server responds with an Upgrade header, the session may or may not be configured to handle it (via one of the plugins). if it isn't, it should just move on and keep the same flow.

on a keep-alive connection though, the next request would be buffered by the parser in an invalid state, because the parser itself wasn't being reset after handling the upgrade payload bits and before bailing out.

Closes #376 (closed)