fix(ngCookie): log warning when attempting to store a non-string value

Previously, non-string values stored in $cookies would be removed, without warning the user, and
causing difficulty debugging. Now, a warning is emitted, so that there is some idea of where the
error is occurring.

Closes angular#6151

src/ngCookies/cookies.js

@@ -45,7 +45,7 @@ angular.module('ngCookies', ['ng']).
      </file>
    </example>
    */
-   factory('$cookies', ['$rootScope', '$browser', function ($rootScope, $browser) {
+   factory('$cookies', ['$rootScope', '$browser', '$log', function ($rootScope, $browser, $log) {
       var cookies = {},
           lastCookies = {},
           lastBrowserCookies,
@@ -98,6 +98,8 @@ angular.module('ngCookies', ['ng']).
             if (angular.isDefined(lastCookies[name])) {
               cookies[name] = lastCookies[name];
             } else {
+              $log.log('$cookies["' + name + '"] must be "string", but value is "' +
+                       typeof value + '"');
               delete cookies[name];
             }
           } else if (value !== lastCookies[name]) {

test/ngCookies/cookiesSpec.js

@@ -46,14 +46,25 @@ describe('$cookies', function() {
 
 
   it('should drop or reset any cookie that was set to a non-string value',
-      inject(function($cookies, $browser, $rootScope) {
+      inject(function($cookies, $browser, $rootScope, $log) {
     $cookies.nonString = [1, 2, 3];
     $cookies.nullVal = null;
     $cookies.undefVal = undefined;
     $cookies.preexisting = function() {};
     $rootScope.$digest();
     expect($browser.cookies()).toEqual({'preexisting': 'oldCookie'});
     expect($cookies).toEqual({'preexisting': 'oldCookie'});
+    $log.reset();
+  }));
+
+
+  it('should log an error when a cookie is set to a non-string value',
+      inject(function($cookies, $browser, $rootScope, $log) {
+    spyOn($log, 'log');
+    $cookies.number = 123;
+    $rootScope.$digest();
+    expect($log.log).toHaveBeenCalledWith('$cookies["number"] must be "string", but value is "number"');
+    $log.reset();
   }));