Increase branch coverage: Array.prototype.join

Branch coverage:
Before: 19/20
After: 20/20

JerryScript-DCO-1.0-Signed-off-by: Mate Dabis mdabis@inf.u-szeged.hu

Author: matedabis

tests/jerry/array-prototype-join.js

@@ -74,3 +74,12 @@ obj_2[3] = 4;
 obj_2.join = Array.prototype.join;
 
 assert (obj_2.join() === "1,2,3");
+
+/* ES v5.1 15.4.4.5.1.
+   Checking behavior when the function's this_argument is undefined */
+try {
+  Array.prototype.join.call(undefined);
+  assert(false);
+} catch (e) {
+  assert(e instanceof TypeError);
+}