fix(input): set ngTrueValue on required checkbox

Fixes angular#5164

Author: Narretz

src/ng/directive/input.js

@@ -1253,9 +1253,9 @@ function checkboxInputType(scope, element, attr, ctrl, $sniffer, $browser, $filt
     element[0].checked = ctrl.$viewValue;
   };
 
-  // Override the standard `$isEmpty` because a value of `false` means empty in a checkbox.
+  // Override the standard `$isEmpty` because an empty checkbox is always set to `false`
   ctrl.$isEmpty = function(value) {
-    return value !== trueValue;
+    return value === false;
   };
 
   ctrl.$formatters.push(function(value) {

test/ng/directive/inputSpec.js

@@ -3623,7 +3623,7 @@ describe('input', function() {
 
 
     it('should be required if false', function() {
-      compileInput('<input type="checkbox" ng:model="value" required />');
+      compileInput('<input type="checkbox" ng-model="value" required />');
 
       browserTrigger(inputElm, 'click');
       expect(inputElm[0].checked).toBe(true);
@@ -3633,6 +3633,16 @@ describe('input', function() {
       expect(inputElm[0].checked).toBe(false);
       expect(inputElm).toBeInvalid();
     });
+
+    it('should not be required with ngTrueValue', function() {
+      compileInput('<input type="checkbox" ng-model="value" required ng-true-value="\'yes\'" />');
+
+      expect(inputElm).toBeInvalid();
+
+      browserTrigger(inputElm, 'click');
+      expect(inputElm[0].checked).toBe(true);
+      expect(inputElm).toBeValid();
+    });
   });