Fix non-rvalue Json::Value assignment operator (should copy, not move)

src/lib_json/json_value.cpp

@@ -518,18 +518,10 @@ Value::~Value() {
   value_.uint_ = 0;
 }
 
-Value& Value::operator=(const Value& other) {
-  swap(const_cast<Value&>(other));
-  return *this;
-}
-
-#if JSON_HAS_RVALUE_REFERENCES
-Value& Value::operator=(Value&& other) {
-  initBasic(nullValue);
+Value& Value::operator=(Value other) {
   swap(other);
   return *this;
 }
-#endif
 
 void Value::swapPayload(Value& other) {
   ValueType temp = type_;