CSW: Use DublinCore as default output schema for GetRecords responses

Currently, we return the native metadata format (ISO/EBrim/...) in case the outputSchema value is unknown. AFAIK, DublinCore is the only common and required metadata encoding that every CSW has to implement.

Therefore, I suggest to change the code to return DC in case the outputSchema attribute doesn't match any known value. This has two benefits:

* The strategy seems closer to the CSW spec. (DC is required to be supported for all application profiles)
* The behaviour matches deegree 2 behaviour better