Fixing Enumerator#peek behaviour. When the Enumerator position is already at the end and method peek is called then JRuby was throwing a NullPointerException. But the correct behaviour should be raise a StopIteration.
Merge pull request #1336 from lucasallan/enumerator-peek

Fixing Enumerator#peek behaviour
Merge with jruby-1_7 for Enumerator#peek fix