zlib: emit "close" if Z_STREAM_END occurs.

In src/node_zlib.cc, Z_OK, Z_STREAM_END and Z_BUF_ERROR has been ignored because they are considered normal statuses.

But Z_STREAM_END must be handled. Even if zlib does not end when it meets Z_STREAM_END, it should at least tell user so that user can stop it.

(Actaully, I believe "end" is better than "close" for this case, but emitting "end" may cause exception because zlib does not allow to write something to itself if zlib._ended is true.)