Fix TestUnbuffer#testUnbufferException failure

Hello! I am a new contributor and actually contributing to open source for the very first time.

I pulled down Hadoop today and when running the tests I encountered a failure with the TestUnbuffer#testUnbufferException test.

The unbuffer code has recently gone through some changes and I believe this test case may have been overlooked. Using today's git commit (659e85e304d070f9908a96cf6a0e1cbafde6a434), and upon running the test case, there is an expect mock for an exception UnsupportedOperationException that is no longer being thrown.

It would appear that a test like this would be valuable so my initial proposed patch did not remove it. Instead, I removed the conditions that were guarding the cast from being able to fire – as was the previous behavior. Now when we encounter an object that doesn't have the UNBUFFERED StreamCapability, it will throw an error as it did prior to the recent changes.

Please review and let me know what you think!