Add _.toLower and _.toUpper.

test/test.js

@@ -20991,7 +20991,9 @@
       'some',
       'startsWith',
       'toInteger',
+      'toLower',
       'toString',
+      'toUpper',
       'trim',
       'trimLeft',
       'trimRight',
@@ -21236,7 +21238,7 @@
     var acceptFalsey = _.difference(allMethods, rejectFalsey);
 
     QUnit.test('should accept falsey arguments', function(assert) {
-      assert.expect(253);
+      assert.expect(255);
 
       var emptyArrays = _.map(falsey, _.constant([]));