Update FullCopyingReader.java

Inherited `Reader` documentation of `close()` method says: «Closing a previously closed stream has no effect.» But when trying to close already closed `FullCopyingReader`, `IOUtils.copyLarge(source, copy)` tries to `read` from already closed `source`, that was closed during previous `close()` call. The read causes `IOException`, such behavior is described in `Reader`'s documentation too.

This update fixes described problem.