fix NSStrings are compared with '=' operator

Author: zhangzifeng02

ios/IJKMediaPlayer/IJKMediaPlayer/IJKMediaPlayback.m

@@ -95,9 +95,6 @@ - (void)setUrl:(NSString *)url
 
     _handled = YES;
 
-    if (url == _url)
-        return;
-
     if (![self.url isEqualToString:url]) {
         _urlChanged = YES;
         _url = url;