Fixed problem in TupleEntrySchemeIterator hasNext()

Fixed problem in TupleEntrySchemeIterator where hasNext() returns different values when called multiple times before calling next(), added platformTest and TupleEntrySchemeIteratorTest.

I feel the TupleEntrySchemeIteratorTest would benefit from a mocking framework like mockito but I didn't want to add the dependency (I'll be happy to change that if preferred).
The add test in platformTest is a bit brittle in my opinion and it might not add so much I mostly added it as it was recommended practise in the "How to contribute"-documentation.