url出现特殊字符处理-- stringByAddingPercentEncodingWithAllowedCharacters

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