[PATCH] fix ssl-bump bypass on intercepted traffic

The SSL-bump bypass code on intercepted HTTPS traffic generates a fake
CONNECT request from the original destination IP:port in an attempt to
trigger a TCP tunnel being opened for the un-bumped data to be
transferred over.

The current implementation breaks in two situations:

1) when IPv6 traffic is intercepted

The URL field generated does not account for the additional []
requirements involved when IPv6+port are combined.

The resulting fake requests look like:

  CONNECT ::1:443 HTTP/1.1
  Host: ::1

.. .which are both invalid, and will fail to parse. Breaking IPv6 HTTPS
interception bypass.

The attached patch resolves this by using Ip::Address::ToURL() function
which was created for the purpose of generating URL hostnames from raw-
IP + port including the bracketing when required.

2) when a non-443 port is being intercepted

The Host: header generated is missing the port and Squid Host: header
validity will reject the outbound

  CONNECT 127.0.0.1:8443 HTTP/1.1
  Host: 127.0.0.1

... this is an invalid request. Squid is currently ignoring the Host
header. However Squid tunnel.cc does make use of peering and may relay
the fake request Host: to upstream peers where we cannot be so sure what
will happen.

The attached patch resolves this issue by re-using the generated IP:port
string for both URL and Host: fields, which preserves teh port in Host:
regardless of value. This also means there is an unnecessary :443 tagged
on for most HTTPS traffic, however the omission of port from the Host:
header is only a MAY and this should not cause any issues.

Amos