Guid handling and explicit null handling

[Onkelborg]

String guid to System.Guid fixed, proper handling of explicit null in json body ( {example: null } )

I haven't done any extensive testing, but these modifications seem to work for me. Use of TypeDescriptor allows Guid strings to be converted to real System.Guid - didn't work before. Checking of the keys in parameters (and use null instead of an empty string) allows for explicit null and the possibility to differentiate between null and empty strings.