tests: updates change user test to cover calling without specifing new user

Author: dresende

test/unit/connection/test-change-user.js

@@ -21,8 +21,18 @@ server.listen(common.fakeServerPort, function(err) {
         assert.ifError(err);
         assert.strictEqual(result[0]['CURRENT_USER()'], 'user_2@localhost');
 
-        connection.destroy();
-        server.destroy();
+        // should keep current user
+        connection.changeUser(function (err) {
+          assert.ifError(err);
+
+          connection.query('SELECT CURRENT_USER()', function (err, result) {
+            assert.ifError(err);
+            assert.strictEqual(result[0]['CURRENT_USER()'], 'user_2@localhost');
+
+            connection.destroy();
+            server.destroy();
+          });
+        });
       });
     });
   });