fix(ui-number-mask): show "0" if model is 0

Show 0, instead of undefined, if the model value has changed to 0.
When ui-number-mask="0" is used and the model changes the value
to 0, then undefined is displayed in the input field.
The cause of the error is, that clearDelimitersAndLeadingZero
does not only clear leading zeros, but also a single 0
(what is equal to the vaue 0). -> Do not strip a single 0.

Closes ##111

src/global/number/number.test.js

@@ -111,6 +111,16 @@ describe('ui-number-mask', function() {
 		});
 	}));
 
+	it('should show zero when the model value is zero and the precision is set to 0', inject(function($rootScope) {
+		var input = TestUtil.compile('<input ng-model="model" ui-number-mask="0">');
+		var model = input.controller('ngModel');
+
+
+		$rootScope.model = 0;
+		$rootScope.$digest();
+		expect(model.$viewValue).toBe('0');
+	}));
+
 	it('should accept negative numbers if "ui-negative-number" is defined', function() {
 		var input = TestUtil.compile('<input ng-model="model" ui-number-mask ui-negative-number>');
 		var model = input.controller('ngModel');

src/helpers.js

@@ -6,9 +6,12 @@ module.exports = m.name;
 
 m.factory('PreFormatters', [function(){
 	function clearDelimitersAndLeadingZeros(value) {
+		if (value === '0') {
+			return '0';
+		}
+
 		var cleanValue = value.replace(/^-/,'').replace(/^0*/, '');
-		cleanValue = cleanValue.replace(/[^0-9]/g, '');
-		return cleanValue;
+		return cleanValue.replace(/[^0-9]/g, '');
 	}
 
 	function prepareNumberToFormatter (value, decimals) {