stringByAddingPercentEscapesUsingEncoding(只对 `#%^{}[]|\"<> 加空格共14个字符编码，不包括”&?”等符号), ios9将淘汰，建议用 stringByAddingPercentEncodingWithAllowedCharacters 方法
stringByAddingPercentEncodingWithAllowedCharacters 需要传一个 NSCharacterSet 对象（关于 NSCharacterSet 这篇 文章说的很好）
如[NSCharacterSet URLQueryAllowedCharacterSet]
URLFragmentAllowedCharacterSet "#%<>[\]^`{|}
URLHostAllowedCharacterSet "#%/<>?@\^`{|}
URLPasswordAllowedCharacterSet "#%/:<>?@[\]^`{|}
URLPathAllowedCharacterSet "#%;<>?[\]^`{|}
URLQueryAllowedCharacterSet "#%<>[\]^`{|}
URLUserAllowedCharacterSet "#%/:<>?@[\]^`
（2）网络访问请求：中文空格字符解码
stringByRemovingPercentEncoding ---- xcode7可能会提示要将stringByAddingPercentEscapesUsingEncoding替换成此方法，要根据是否是解码来区分
代替 stringByAddingPercentEscapesUsingEncoding
let customAllowedSet = NSCharacterSet(charactersInString:"`#%^{}\"[]|\\<> ").invertedSet
NSString *urlStr = [urlStr stringByAddingPercentEncodingWithAllowedCharacters:[[NSCharacterSet characterSetWithCharactersInString:@"?!@#$^&%*+,:;='\"`<>()[]{}/\\| "] invertedSet]];