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