Added CONNECT method rejection.

No valid CONNECT requests are expected to appear within nginx, since it
is not a forward proxy.  Further, request line parsing will reject
proper CONNECT requests anyway, since we don't allow authority-form of
request-target.  On the other hand, RFC 7230 specifies separate message
length rules for CONNECT which we don't support, so make sure to always
reject CONNECTs to avoid potential abuse.