Make HttpURLConnection streams throw IOException after close()

This is to revert to prior behavior found in earlier releases:
OkHttp changed to throw IllegalStateException.

HttpEngine has also been changed to explicitly close the request
body on disconnect() to ensure that these exceptions are thrown
at the expected moment (e.g. on write() on a closed, buffered
stream).

Additional close() checks have been added to okio classes and
tests have been added. The tests are probes for interesting
examples, not complete.