Fix ForEachIterable error when calling iterator method without 'this'

[AndrewMonteith]

Problem

The ForEachIterable check will NPE when generating a fix for an instance where the method returning the iterator is a member function, (ie called with this as the receiver). This causes getReceiver to return null, as expected, but then will trigger a NPE from state.getSourceForNode. The other two cases getReceiver can return null are for static methods, this will never occur though since the earlier ITERATOR matcher does not match static methods.

PR Contribution

If getReceiver returns null then the fix will iterate over 'this' since getReceiver is returning null for getReceiver(this.iterator()) so in reality should return this. The new case, and associated fix, can be seen in the unit test.