fix golint issues in core/mapping (#497)

core/mapping/valuer.go

@@ -1,13 +1,17 @@
 package mapping
 
 type (
+	// A Valuer interface defines the way to get values from the underlying object with keys.
 	Valuer interface {
+		// Value gets the value associated with the given key.
 		Value(key string) (interface{}, bool)
 	}
 
+	// A MapValuer is a map that can use Value method to get values with given keys.
 	MapValuer map[string]interface{}
 )
 
+// Value gets the value associated with the given key from mv.
 func (mv MapValuer) Value(key string) (interface{}, bool) {
 	v, ok := mv[key]
 	return v, ok