Overwrite default encoding for json

Class Body in method parseContentType is assuming (basing on RFC 2068) that ISO-8859-1 is default charset in case none is provided... while RFC-4627 (https://tools.ietf.org/html/rfc4627) says "JSON text SHALL be encoded in Unicode. The default encoding is UTF-8."

Possible resolutions are to change default encoding for JSON or to allow custom encoding using env variables or some parameter