addr: fix to_http_authority panic with IPv6

[hawkw]

Currently, the Addr::to_http_authority method panics when called on a
SocketAddr which is an IPv6 address with port 80. This method does not
panic when called with IPv4 addresses, or with IPv6 addresses whose
ports are not port 80. This was initially caught by oss-fuzz; see
here for details.

The panic occurs because when an IPv6+ address occurs in an authority,
it must be within square brackets, as per RFC3986, Section 3.2. The
square brackets distinguish between colons in the IPv6 address and the
colon separating the address and port. When the SocketAddr's port is
not port 80, we format it including the port, and the fmt::Display
output from IPv6 SocketAddrs includes the square brackets as expected.

However, when the socket's port is port 80, we have special logic for
eliding the port from the authority. This works fine for IPv4, where we
can just call addr.ip().to_string() to nicely format the address.
However, with IPv6 addresses, this only formats the address itself,
not the square brackets. According to RFC3986, square brackets are
mandatory for all IPv6 addresses, even when port 80 is elided.

This branch fixes the panic by changing Addr::to_http_authority to
include square brackets when formatting IPv6 SocketAddrs with port 80.

I've also improved on @olix0r's original test cases from
dbf898a to include IPv6 addrs with and without
shorthand, and to test ports that are and are not port 80. These tests
helped catch the panic, and may be useful to guard against future
regressions.

Fixes linkerd/linkerd2#6020

Signed-off-by: Eliza Weisman eliza@buoyant.io