BUG/MINOR: mux-h2: CONTINUATION in closed state must always return GOAWAY

BUG/MINOR: mux-h2: CONTINUATION in closed state must always return GOAWAY

Since we now support CONTINUATION frames, we must take care of properly
aborting the connection when they are sent on a closed stream. By default
we'd get a stream error which is not sufficient since the compression
context is modified and unrecoverable.

More info in this discussion :

   https://mailarchive.ietf.org/arch/msg/httpbisa/azZ1jiOkvM3xrpH4jX-Q72KoH00

This needs to be backported to 1.9 and possibly to 1.8 (less important there).