Fix progressCallback not get called when call downloadFile on iOS platform

[zhima]

On iOS platform, when I call downloadFile method without setting the begin callback param, the progress callback not get called anyway.

In Downloader.m file, progressCallback get called only when statusCode equal to 200, and statusCode is assigned value only when there is beginCallback.

So I put the statusCode's assigning before beginCallback's determination, so that statusCode will be assigned value without beginCallback.

[siisee11]

Thanks.