fix(input): set ngTrueValue on required checkbox

Fixes angular#5164

src/ng/directive/input.js

@@ -1338,9 +1338,11 @@ 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 the $viewValue of an empty checkbox is always set to `false`
+  // This is because of the parser below, which compares the `$modelValue` with `trueValue` to convert
+  // it to a boolean.
   ctrl.$isEmpty = function(value) {
-    return value !== trueValue;
+    return value === false;
   };
 
   ctrl.$formatters.push(function(value) {

test/ng/directive/inputSpec.js

@@ -2255,6 +2255,22 @@ describe('input', function() {
   });
 
 
+  it('should render the $viewValue when $modelValue is empty', function() {
+    compileInput('<input type="text" ng-model="value" />');
+
+    var ctrl = inputElm.controller('ngModel');
+
+    ctrl.$modelValue = null;
+
+    expect(ctrl.$isEmpty(ctrl.$modelValue)).toBe(true);
+
+    ctrl.$viewValue = 'abc';
+    ctrl.$render();
+
+    expect(inputElm.val()).toBe('abc');
+  });
+
+
   describe('pattern', function() {
 
     it('should validate in-lined pattern', function() {