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.

Specifically, the values `0` (the number zero), `null`, `NaN` and `''` (the
empty string) used to considered valid (passing) and they are now considered
invalid (failing). The value `undefined` was treated similarly to a pending
asynchronous validator, causing the validation to be pending. `undefined` is
also now considered invalid.

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 be 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

test/ng/directive/ngModelSpec.js

@@ -848,18 +848,12 @@ describe('ngModel', function() {
       });
 
       it('should treat all responses as boolean for synchronous validators', function() {
-        // var curry = function(v) {
-        //   return function() {
-        //     return v;
-        //   };
-        // };
-
-        var expectValid = function(v, e) {
+        var expectValid = function(value, expected) {
           ctrl.$modelValue = undefined;
-          ctrl.$validators.a = valueFn(v);
+          ctrl.$validators.a = valueFn(value);
 
           ctrl.$validate();
-          expect(ctrl.$valid).toBe(e);
+          expect(ctrl.$valid).toBe(expected);
         };
 
         // False tests