MULE-6833: GZip transformer failing

GZipCompressTransformer serializes objects (e.g. Strings) into byte arrays before compressing. Its counterpart, the GZipUncompressTransformer receives a byte array, decompresses it and only deserializes back into an object if the return class is explicitely set on the transformer. Now the GZipUncompressTransformer tries to "guess" if the decompressed byte array was serialized by trying to deserialize it and checking if it failed. If the deserialization was successful it returns the resulting object, otherwise, it returns the decompressed byte array.

MULE-6833: GZip transformer failing

- Added missing license header.
(cherry picked from commit 99b7507eaddc16847dc9dd8b65c6808b39a0c26b)

Conflicts:
	core/src/main/java/org/mule/transformer/compression/GZipUncompressTransformer.java
	core/src/test/java/org/mule/transformer/compression/GZipTransformerStreamTestCase.java