Fix possible thread safe issue for ResponseWriter, issue #106:

- Problem: ResponseWriter are loaded by ServiceLoader so that only one instance exists. Then the singleton class is modified which might be a problem if two requests are handled by the same ResponseWriter at the same time with different content types.
- Solution: Add factories for each ResponseWriter implementation and load these factories in the ResponseWriterRepository. If a ResponseWriter is requested, the corresponding factory creates a new ResponseWriter implementation.