downloaderCompletedTask: never called if completed successfully (includes simple fix)

[LouisStAmour]

Just change the _markComplete method in AQLowMemoryDownloadHelper.m as follows:

- (void) _markComplete
{
    self.complete = YES;
    if (self.asyncDelegate != nil) {
        [self.asyncDelegate performSelector:@selector(downloaderCompletedTask:) withObject:self];
    }
}

And voila, it appears to work fine now. (I haven't tried testing multiple downloads at one time, etc., so you may need to experiment. In addition, it seems like downloaderCompletedTask is never called by the existing code, so I'll also want to add this line or another, to indicate failure. ymmv.)