Consider 'background' attribute as unsafe

When loading a message, background attributes will be disabled if the
user as chosen to not automatically load external images.

Fixes #2437

UI/MailPartViewers/UIxMailPartHTMLViewer.m

@@ -542,7 +542,8 @@ static NSData* _sanitizeContent(NSData *theData)
                   else
                     skipAttribute = YES;
                 }
-              else if (([name isEqualToString: @"data"]
+              else if ([name isEqualToString: @"background"] ||
+                       ([name isEqualToString: @"data"]
                         || [name isEqualToString: @"classid"])
                        && [lowerName isEqualToString: @"object"])
                 {