Consolidate helper method into one place

* The method `trimURLSpacing` is defined in both OneSignalCoreHelper and OneSignalAttachmentHandler
* Let's just use it in one place, the OneSignalCoreHelper, as this method is also used elsewhere.

Author: nan-li

iOS_SDK/OneSignalSDK/OneSignalExtension/OneSignalAttachmentHandler.m

@@ -209,7 +209,7 @@ + (void)addAttachments:(OSNotification*)notification
     let unAttachments = [NSMutableArray new];
 
     for(NSString* key in notification.attachments) {
-        let URI = [OneSignalAttachmentHandler trimURLSpacing:[notification.attachments valueForKey:key]];
+        let URI = [OneSignalCoreHelper trimURLSpacing:[notification.attachments valueForKey:key]];
 
         let nsURL = [NSURL URLWithString:URI];
 
@@ -306,14 +306,6 @@ + (UNNotificationAction *)createActionForButton:(NSDictionary *)button {
     }
 }
 
-+ (NSString*)trimURLSpacing:(NSString*)url {
-    if (!url)
-        return url;
-    
-    return [url stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
-}
-
-
 /*
  Synchroneously downloads an attachment
  On success returns bundle resource name, otherwise returns nil