Warn on bracketed IPv6 address with a prefix length

[taketo1113]

Summary

IPAddr.new("[2001:db8::]/32") is currently accepted and treated the same as IPAddr.new("2001:db8::/32").
However, square brackets are a delimiter for embedding an address in a larger syntax such as host:port, and no notation is defined for a prefix length after the closing bracket.

This change prints a warning for this form, saying that it will raise IPAddr::InvalidAddressError in a future version. The result is unchanged for now.

Why [2001:db8::]/32 is invalid

RFC 5952: Section 6 defines how an IPv6 address is combined with a port number in text. It says the [] style from RFC 3986 should be used by default (e.g. [2001:db8::1]:80), and that URIs containing IPv6 literals must follow RFC 3986.

In other words, square brackets wrap the address only, as a delimiter for embedding it in a larger syntax.
The RFC does not define a form in which a prefix length follows the closing bracket.
The valid forms are [2001:db8::] (bare address in bracketed style) and 2001:db8::/32 (address with a prefix length, no brackets).

Changes

This Pull Request adds a warning that is printed when a bracketed IPv6 address is followed by a prefix length or a mask.

Behavior

Input	Before	After
"[2001:db8::]"	valid	valid (no change)
"2001:db8::/32"	valid	valid (no change)
"[2001:db8::]/32"	accepted	accepted, with a warning

Example:

irb> IPAddr.new("[2001:db8::]/32")
(irb):1: warning: bracketed IPv6 address with a prefix length is an invalid address and will raise IPAddr::InvalidAddressError in a future version: [2001:db8::]/32
=> #<IPAddr: IPv6:2001:0db8:0000:0000:0000:0000:0000:0000/ffff:ffff:0000:0000:0000:0000:0000:0000>