Description:
------------
IPv6 addresses with a single abbreviated field are being validated 
inconsistently.

For example the following IPv6 address is marked as valid:

    1::2:3:4:5:6:7

but the following two IPv6 addresses are marked as invalid:

    ::1:2:3:4:5:6:7
    1:2:3:4:5:6:7::

How should they be validated? The authority on the text representation of IPv6 
addresses is RFC 4291, which allows all three examples. However the recently-
published RFC 5952 deprecates a format where a single field is abbreviated in 
this way, although it clearly states that such addresses must be accepted if 
presented. RFC 5952 defines a recommended formatting for *outputting* IPv6 
addresses, not for validating incoming ones.

RFC 3986 also allows this syntax, but RFC 5321 does not (although like RFC 5952 
it acknowledges the ultimate authority of RFC 4291).

The point of this bug report, however, is to point out the inconsistent 
validation of these addresses.

I believe they should all be allowed (as per RFCs 4291 and 3986). Warning: this 
might result in filter_var allowing some email addresses that have IPv6 address 
literals that are OK according to RFC 4291 but not RFC 5321.

Test script:
---------------
<?php
function isIP($address) {
  $valid = filter_var($address, FILTER_VALIDATE_IP);
  echo "$address is", ($valid ? '' : ' not'), ' valid<br/>';
}

isIP('1:2:3::4:5:6:7');
isIP('::1:2:3:4:5:6:7');
isIP('1:2:3:4:5:6:7::');
?>

Expected result:
----------------
1:2:3::4:5:6:7 is valid
::1:2:3:4:5:6:7 is valid
1:2:3:4:5:6:7:: is valid

Actual result:
--------------
1:2:3::4:5:6:7 is valid
::1:2:3:4:5:6:7 is not valid
1:2:3:4:5:6:7:: is not valid