Get the output stream. This method will be called after all the
writer interceptors are called and written entity is buffered
into the buffer or the buffer exceeds.
contentLength - the size of the buffered entity or -1 if the entity exceeded the maximum buffer
size or if the buffering is disabled.
the adapted output stream into which the serialized entity should be written. May return null
which will cause ignoring the written entity (in that case the entity will
still be written by message body writers
but the output will be ignored).