fix(ngModel): treat synchronous validators as boolean always

Change synchronous validators to convert the return to boolean value.
Prevent unexpected behavior when returning `undefined`.

Closes angular#14734
BREAKING CHANGE: Previously, only a literal `false` return would resolve as the
synchronous validator failing. Now, all traditionally false JavaScript values
are treated as failing the validator, as one would naturally expect.

To migrate, make sure your synchronous validators are returning either a
literal `true` or a literal `false` value. For most code, we expect this to
already be the case. Only a very small subset of projects will affected.

Namely, anyone using `undefined` or any falsy value as a return will now see
their validation failing, whereas previously falsy values other than `undefined`
would have been seen as passing and `undefined` would have been seen as pending.

Author: BobChao87

src/ng/directive/ngModel.js

@@ -603,8 +603,7 @@ NgModelController.prototype = {
     function processSyncValidators() {
       var syncValidatorsValid = true;
       forEach(that.$validators, function(validator, name) {
-        var result = validator(modelValue, viewValue);
-        // var result = Boolean(validator(modelValue, viewValue));
+        var result = Boolean(validator(modelValue, viewValue));
         syncValidatorsValid = syncValidatorsValid && result;
         setValidity(name, result);
       });

test/ng/directive/ngModelSpec.js

@@ -847,43 +847,7 @@ describe('ngModel', function() {
         expect(ctrl.$valid).toBe(true);
       });
 
-      // See also the test on L886.
-      fit('should demonstrate how synchronous validators work now', function() {
-        var curry = function(v) {
-          return function() {
-            return v;
-          };
-        };
-
-        var tester = function(v, e) {
-          ctrl.$modelValue = undefined;
-          ctrl.$validators.a = curry(v);
-
-          ctrl.$validate();
-          expect(ctrl.$valid).toBe(e);
-        };
-
-        // False tests
-        tester(false, false);
-        tester(undefined, undefined);
-        tester(null, true);
-        tester(0, true);
-        tester(NaN, true);
-        tester('', true);
-
-        // True tests
-        tester(true, true);
-        tester(1, true);
-        tester('0', true);
-        tester('false', true);
-        tester([], true);
-        tester({}, true);
-      });
-
-      // Flip this test with the one above (L851) as well as line L606 with L607 in `ngModel.js`
-      // The differences between the two would likely being a breaking change
-      // Though the feature it is breaking was undocumented.
-      xit('should demonstrate how synchronous validators will work after this change', function() {
+      it('should treat all responses as boolean for synchronous validators', function() {
         var curry = function(v) {
           return function() {
             return v;