Fix use of static RDFFormat.valueOf function

For some reason, the passed in RDFFormat was being converted to a string instead of being stored, which required the use of the deprecated (in Sesame-2.8) RDFFormat.valueOf function to get the constant back.

Although it is better just to store the format directly than to have to regenerate it, it also has implications for third-party formats which are created but may not have been registered with RDFFormat and hence may not come back from RDFFormat.valueOf