feat(ngMaxlength): add support for disabling max length limit

Previously, setting the maxlength to a negative number, would make all
input values invalid (since their length should be less than maxlength,
which is impossible).
This commit changes the behaviour of maxlength/ngMaxlength, effectively
disabling the maxlength validation (always returning true) when maxlength
is set to a negative number. This is more inline to how the HTML5
`maxlength` attribute works (both in browsers and according to the spec:
http://dev.w3.org/html5/spec-preview/attributes-common-to-form-controls.html#attr-fe-maxlength).

Related to angular#9874
Closes angular#9995

Author: gkalpak

src/ng/directive/input.js

@@ -2636,7 +2636,7 @@ var maxlengthDirective = function() {
         ctrl.$validate();
       });
       ctrl.$validators.maxlength = function(modelValue, viewValue) {
-        return ctrl.$isEmpty(modelValue) || viewValue.length <= maxlength;
+        return (maxlength < 0) || ctrl.$isEmpty(modelValue) || (viewValue.length <= maxlength);
       };
     }
   };

test/ng/directive/inputSpec.js

@@ -2326,6 +2326,16 @@ describe('input', function() {
       expect(inputElm).toBeValid();
     });
 
+    it('should accept values of any length when maxlength is negative', function() {
+      compileInput('<input type="text" ng-model="value" ng-maxlength="-1" />');
+
+      changeInputValueTo('');
+      expect(inputElm).toBeValid();
+
+      changeInputValueTo('aaaaaaaaaa');
+      expect(inputElm).toBeValid();
+    });
+
     it('should listen on ng-maxlength when maxlength is observed', function() {
       var value = 0;
       compileInput('<input type="text" ng-model="value" ng-maxlength="max" attr-capture />');