fix($parser): string concatination with undefined model

Closes #988

src/ng/parse.js

@@ -5,7 +5,15 @@ var OPERATORS = {
     'true':function(){return true;},
     'false':function(){return false;},
     undefined:noop,
-    '+':function(self, locals, a,b){a=a(self, locals); b=b(self, locals); return (isDefined(a)?a:0)+(isDefined(b)?b:0);},
+    '+':function(self, locals, a,b){
+      a=a(self, locals); b=b(self, locals);
+      if (isDefined(a)) {
+        if (isDefined(b)) {
+          return a + b;
+        }
+        return a;
+      }
+      return isDefined(b)?b:undefined;},
     '-':function(self, locals, a,b){a=a(self, locals); b=b(self, locals); return (isDefined(a)?a:0)-(isDefined(b)?b:0);},
     '*':function(self, locals, a,b){return a(self, locals)*b(self, locals);},
     '/':function(self, locals, a,b){return a(self, locals)/b(self, locals);},

test/ng/interpolateSpec.js

@@ -15,6 +15,7 @@ describe('$interpolate', function() {
 
   it('should suppress falsy objects', inject(function($interpolate) {
     expect($interpolate('{{undefined}}')()).toEqual('');
+    expect($interpolate('{{undefined+undefined}}')()).toEqual('');
     expect($interpolate('{{null}}')()).toEqual('');
     expect($interpolate('{{a.b}}')()).toEqual('');
   }));
@@ -55,6 +56,31 @@ describe('$interpolate', function() {
   }));
 
 
+  it('should ignore undefined model', inject(function($interpolate) {
+    expect($interpolate("Hello {{'World' + foo}}")()).toEqual('Hello World');
+  }));
+
+
+  it('should ignore undefined return value', inject(function($interpolate, $rootScope) {
+    $rootScope.foo = function() {return undefined};
+    expect($interpolate("Hello {{'World' + foo()}}")($rootScope)).toEqual('Hello World');
+  }));
+
+
+  describe('provider', function() {
+    beforeEach(module(function($interpolateProvider) {
+      $interpolateProvider.startSymbol('--');
+      $interpolateProvider.endSymbol('--');
+    }));
+
+    it('should not get confused with same markers', inject(function($interpolate) {
+      expect($interpolate('---').parts).toEqual(['---']);
+      expect($interpolate('----')()).toEqual('');
+      expect($interpolate('--1--')()).toEqual('1');
+    }));
+  });
+
+
   describe('parseBindings', function() {
     it('should Parse Text With No Bindings', inject(function($interpolate) {
       var parts = $interpolate("a").parts;