LibJS: Treat missing arg in Array.prototype.includes() as undefined

Author: https://github.com/linusg
Commit: 5db9becc4a
Pull-request: https://github.com/SerenityOS/serenity/pull/2324
Reviewed-by: https://github.com/Dexesttp

Libraries/LibJS/Tests/Array.prototype.includes.js

@@ -5,6 +5,8 @@ try {
 
     var array = ['hello', 'friends', 1, 2, false];
 
+    assert([].includes() === false);
+    assert([undefined].includes() === true);
     assert(array.includes('hello') === true);
     assert(array.includes(1) === true);
     assert(array.includes(1, -3) === true);