Fix: assertCount will no longer cause a rewind error when given a generator (as per #2149)

[Nessworthy]

Hi, first attempt at a pull request, attempting to tackle #2149!

The issue was found to be in two areas in Framework\Constraint\Count::getCountOf:

1. Firstly, where a generator had already been iterated at least once, iterator_count() attempted to rewind the generator object which fired off the error.
2. Secondly, where a generator had not been iterated through at all, after the iterator_count() call exhausted it, the method would then attempt to rewind the generator object to set it back to the original position, also causing the error.

The fix attempts to get around this by looping through the generator itself and counting up the times it was iterated. Non-generators are not affected by this change.

A new test was added to cover both a fresh generator and a generator which had been partially exhausted already.

The caveats from this are that the generator will always be fully exhausted as a result of the assertion, and that counting a partially exhausted generator will only be able to return the count from its current position to the end.

Feedback is appreciated!

[sebastianbergmann]

Merged manually into 5.7, 6.0, and master.