Add ImmutableMultivaluedMap.toString() for nice debugging

When writing a service that needs dynamic query parameters, it's nice to be able to log them out. Unfortunately the parameters object returned by `UriInfo.getQueryParameters()` is an `ImmutableMultivaluedMap` which doesn't have a `toString()` method.

This patch adds that so that a developer can log out the parameters easily.

Edit: Filed [JERSEY-2563](https://java.net/jira/browse/JERSEY-2563)