REST server should assume keys are text/plain by default

For historical reasons, the REST server assumes keys are stored as java.lang.String when the Key-Content-Type header is not sent.

Assuming a cache is configured to store textual content, e.g. text/plain, it will end up storing keys as java.lang.String and values as UTF-8 byte[].

This cause issues with interoperability with Hot Rod clients dealing with text content, since the StringMarshaller always deal with byte[].

Textual encodings (including json and xml) should be always stored as byte[] for maximum interoperability. Storing keys or values as java.lang.String should be reserved to application/x-java-object encoding.