iOS原生请求 post 参数包含 & 失败的解决方案

最近自己用NSMutableURLRequest 写post请求的时候，遇到的问题：

post的某一个参数里面包含 & 由于默认的url编码不会把 & 转义，所以导致请求的时候，把 & 当成了拼接参数的符号了，导致把参数拆分并请求失败

失败的原因是：

请求的时候url是用 & 符号拼接的，所以url的编码是不会把 & 转义的，但是参数里面如果包含 & 就需要想办法转义了。 平时之所以没遇到这个问题，是因为AFNetworking已经帮我们做了这部分工作。

解决方案：

确保 参数字典转成json的时候，保证每一个参数里面都不包含 & ，可以手动把 & 转义成 %26 ，也可以用网上的方案统一编码。 请求的时候，把 json 正常转成 data 就OK了。

AFNetworking 处理类似问题的解决方案是：

思路是不用系统的字典转json，而是自己用for循环把每一个字典的字段都用 & 收到拼接下，并且把每个字段值的特殊字符都转义了。

/**
 Returns a percent-escaped string following RFC 3986 for a query string key or value.
 RFC 3986 states that the following characters are "reserved" characters.
    - General Delimiters: ":", "#", "[", "]", "@", "?", "/"
    - Sub-Delimiters: "!", "$", "&", "'", "(", ")", "*", "+", ",", ";", "="

 In RFC 3986 - Section 3.4, it states that the "?" and "/" characters should not be escaped to allow
 query strings to include a URL. Therefore, all "reserved" characters with the exception of "?" and "/"
 should be percent-escaped in the query string.
    - parameter string: The string to be percent-escaped.
    - returns: The percent-escaped string.
 */
NSString * AFPercentEscapedStringFromString(NSString *string) {
    static NSString * const kAFCharactersGeneralDelimitersToEncode = @":#[]@"; // does not include "?" or "/" due to RFC 3986 - Section 3.4
    static NSString * const kAFCharactersSubDelimitersToEncode = @"!$&'()*+,;=";

    NSMutableCharacterSet * allowedCharacterSet = [[NSCharacterSet URLQueryAllowedCharacterSet] mutableCopy];
    [allowedCharacterSet removeCharactersInString:[kAFCharactersGeneralDelimitersToEncode stringByAppendingString:kAFCharactersSubDelimitersToEncode]];

    // FIXME: https://github.com/AFNetworking/AFNetworking/pull/3028
    // return [string stringByAddingPercentEncodingWithAllowedCharacters:allowedCharacterSet];

    static NSUInteger const batchSize = 50;

    NSUInteger index = 0;
    NSMutableString *escaped = @"".mutableCopy;

    while (index < string.length) {
        NSUInteger length = MIN(string.length - index, batchSize);
        NSRange range = NSMakeRange(index, length);

        // To avoid breaking up character sequences such as 👴🏻👮🏽
        range = [string rangeOfComposedCharacterSequencesForRange:range];

        NSString *substring = [string substringWithRange:range];
        NSString *encoded = [substring stringByAddingPercentEncodingWithAllowedCharacters:allowedCharacterSet];
        [escaped appendString:encoded];

        index += range.length;
    }

    return escaped;
}