NSString中如果包括中文字符，在转换为NSURL时得到的值为nil
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解决办法：
NSString *urlString = [NSString stringWithFormat:@"http://api.openweathermap.org/data/2.5/weather?q=%@&units=imperial",cityName];
urlString = [urlString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSURL *url = [NSURL URLWithString:urlString];
NSLog(@"%@", url);
urlString = [urlString stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSURL *url = [NSURL URLWithString:urlString];
NSLog(@"%@", url);