Fix specializer for null surrogate value.
Fix specializer for null surrogate value.

When YASON:*PARSE-JSON-NULL-AS-KEYWORD* is non-NIL, the result of parsing
"null" is a keyword, not a symbol in the yason package, so the encoding
method needs to be specialized on the keyword.

hanshuebner/yason,hanshuebner/yason,hanshuebner/yason