Support JSON::Fragment with the old JSON encoder

Unfortunately, we can't add it to the case statement because the class
might not exist if json hasn't been upgraded.

Co-authored-by: Jean Boussier <[email protected]>

Author: etiennebarrie

activesupport/lib/active_support/json/encoding.rb

@@ -85,6 +85,7 @@ def encode(value)
           # to +object.as_json+, not any of this method's recursive +#as_json+
           # calls.
           def jsonify(value)
+            return value if value.class.name == "JSON::Fragment"
             case value
             when String, Integer, Symbol, nil, true, false
               value