Add ISO19115-3 to ISO19139 conversion and fix date with nil reason

This PR propose some improvements to the conversion from/to ISO19115-3:
* Rename file to match encoding standard name ie. 19115-to-19115-1 to 19139-to-19115-3
* Add reverse conversion back to ISO19139 - first draft (useful for compatibility with CSW servers)
* Fix date which were swallowed when having nilReason attribute with whatever values


Thanks to @davinciagf for testing.