* Expose getContentTypeString for use by tests
* Use it to get constant arguments to @Produces annotations
* Note that text/html is xss-vulnerable (I have no idea how it ever came to expect exactly text/plain)
In a real-world situation this type would be defined in an imported jar, but since here it is defined in a stub the getADeclaredContentType routine can see it is defined as an empty string in the stubbed implementation. Filter these out so the test more closely resembles the real situation.
