hyper should validate header fields on a header block with the CONNECT method #319

RFC 7540 § 8.3:

In HTTP/2, the CONNECT method is used to establish a tunnel over a
single HTTP/2 stream to a remote host for similar purposes. The HTTP
header field mapping works as defined in Section 8.1.2.3 ("Request
Pseudo-Header Fields"), with a few differences. Specifically:

• The ":method" pseudo-header field is set to "CONNECT".
• The ":scheme" and ":path" pseudo-header fields MUST be omitted.
• The ":authority" pseudo-header field contains the host and port to
  connect to (equivalent to the authority-form of the request-target
  of CONNECT requests (see [RFC7230], Section 5.3)).

A CONNECT request that does not conform to these restrictions is
malformed (Section 8.1.2.6).

Currently hyper doesn’t do anything to check this. We should add a check in utilities.py (probably somewhere in _reject_pseudo_header_fields()) that we conform to these restrictions.