BUG/MINOR: h2: ensure we can never send an RST_STREAM in response to an RST_STREAM

BUG/MINOR: h2: ensure we can never send an RST_STREAM in response to an RST_STREAM

There are some corner cases where this could happen by accident. Since
the spec explicitly forbids this (RFC7540#5.4.2), let's add a test in
the two only functions which make the RST to avoid this. Thanks to user
klzgrad for reporting this problem. Usually it is expected to be harmless
but may result in browsers issuing a warning.

This fix must be backported to 1.8.