Date.protoype.toJSON should look for toISOString once (#3996)

The toJSON method should be searched on the `this` object
only once. In case of normal objects there were no problems
however when a Proxy object is used as the `this` argument
for the Date.prototype.toJSON method the Proxy's `get` method
was invoked twice.

JerryScript-DCO-1.0-Signed-off-by: Peter Gal pgal.usz@partner.samsung.com

tests/jerry/date-tojson.js

@@ -0,0 +1,22 @@
+// Copyright JS Foundation and other contributors, http://js.foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+var obj = {
+    toISOString: function() { return "RESULT-ToISOString"; },
+    valueOf: function() { return "RESULT-valueOf"; }
+};
+
+var result = Date.prototype.toJSON.call(obj);
+
+assert(result === "RESULT-ToISOString");