Fixed JERSEY-2563: Missing ImmutableMultivaluedMap.toString().

- Added ImmutableMultivaluedMap toString, equals and hasCode method implementations.

Change-Id: If7e9eab61d9bf1a2811740a22b1d68d1a170feaa
Signed-off-by: Marek Potociar <marek.potociar@oracle.com>
Merge "Fixed JERSEY-2563: Missing ImmutableMultivaluedMap.toString()."