BUG/MINOR: mux-h1: Do h2 upgrade only on the first request

When a request is received, if the h2 preface is matched, an implicit upgrade
from h1 to h2 is performed. This must only be done for the first request on a
connection. But a test was missing to unsure it is really the first request.

This patch must be backported to 2.0.
1 file changed