Unify appearance of avatars for undefined and unknown users

When calling the jQuery avatar plugin with a user that did not exist
(that is, users for which "/avatar/{user}/{size}" return a JSON response
with an empty "displayname" value) "?" on a grey background was shown.
However, if the jQuery avatar plugin was called with an undefined
JavaScript value then "?" was shown on a bluish background. This commit
unifies both cases to use the grey background.

The unit tests were also modified to ensure that the grey background is
used in both cases.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>

core/js/jquery.avatar.js

@@ -73,6 +73,7 @@
 				user = this.data('user');
 			} else {
 				this.imageplaceholder('?');
+				this.css('background-color', '#b9b9b9');
 				return;
 			}
 		}

core/js/tests/specs/jquery.avatarSpec.js

@@ -62,10 +62,12 @@ describe('jquery.avatar tests', function() {
 
 	it('undefined user', function() {
 		spyOn($div, 'imageplaceholder');
+		spyOn($div, 'css');
 
 		$div.avatar();
 		
 		expect($div.imageplaceholder).toHaveBeenCalledWith('?');
+		expect($div.css).toHaveBeenCalledWith('background-color', '#b9b9b9');
 	});
 
 	describe('no avatar', function() {
@@ -86,6 +88,7 @@ describe('jquery.avatar tests', function() {
 
 		it('show placeholder for non existing user', function() {
 			spyOn($div, 'imageplaceholder');
+			spyOn($div, 'css');
 			$div.avatar('foo');
 
 			fakeServer.requests[0].respond(
@@ -97,6 +100,7 @@ describe('jquery.avatar tests', function() {
 			);
 
 			expect($div.imageplaceholder).toHaveBeenCalledWith('foo', '?');
+			expect($div.css).toHaveBeenCalledWith('background-color', '#b9b9b9');
 		});
 
 		it('show no placeholder', function() {