Add support for non-flushing OutputStream to StreamUtils

[jhoeller]

StreamUtils comes with nonClosing decoration methods already, based on an internal NonClosingOutputStream class which suppresses close() calls. We can easily add support for a nonFlushing method, applying an internal subclass of NonClosingOutputStream that also suppresses flush() calls. Also, we can detect whether a given OutputStream is non-closing/flushing already and avoid unnecessary decoration in that case.