We should apply the same restrictions on the list of allowed characters inside
H/2 pseudo-headers as we do for H/1. This error is translated into the
headers we send to a backend over H/1.

Failure to do so could permit various exploits against a backend not handling
malformed H/1 requests.
Signed-off-by: Asad Sajjad Ahmed <asadsa@varnish-software.com>