Restore 2.0  behavior for `ShouldThrow<AggregateException>`

Makes `AggregateExceptionExtractor` not extract exceptions if the assertion is against AggregateException (or subclass of AggregateException).  This allows assertions against the thrown exception and its inner exceptions just as in FA 2.0.  The 2.1+ functionality of extracting AggregateExceptions is preserved for all other exception types.

This seemed to be the least change with the least user impact while 'just working' in the issue scenario.

Should fix Issue #62