GzipCodec NPE upon reset with IBM JDK

The GzipCodec will NPE upon reset after finish when the native zlib codec is not loaded. When the native zlib is loaded the codec creates a CompressorOutputStream that doesn't have the problem, otherwise, the GZipCodec uses GZIPOutputStream which is extended to provide the resetState method. Since IBM JDK 6 SR9 FP2 including the current JDK 6 SR10, GZIPOutputStream#finish will release the underlying deflater, which causes NPE upon reset. This seems to be an IBM JDK quirk as Sun JDK and OpenJDK doesn't have this issue.