BUG/MINOR: mux-h2: headers-type frames in HREM are always a connection error

BUG/MINOR: mux-h2: headers-type frames in HREM are always a connection error

There are incompatible MUST statements in the HTTP/2 specification. Some
require a stream error and others a connection error for the same situation.
As discussed in the thread below, let's always apply the connection error
when relevant (headers-like frame in half-closed(remote)) :

  https://mailarchive.ietf.org/arch/msg/httpbisa/pOIWRBRBdQrw5TDHODZXp8iblcE

This must be backported to 1.9, possibly to 1.8 as well.