fix golint issues in core/mapping (#497)

core/mapping/jsonunmarshaler.go

@@ -10,10 +10,12 @@ const jsonTagKey = "json"
 
 var jsonUnmarshaler = NewUnmarshaler(jsonTagKey)
 
+// UnmarshalJsonBytes unmarshals content into v.
 func UnmarshalJsonBytes(content []byte, v interface{}) error {
 	return unmarshalJsonBytes(content, v, jsonUnmarshaler)
 }
 
+// UnmarshalJsonReader unmarshals content from reader into v.
 func UnmarshalJsonReader(reader io.Reader, v interface{}) error {
 	return unmarshalJsonReader(reader, v, jsonUnmarshaler)
 }