這樣可以產生一個 http method 為 POST，http body 為 multipart/form-data 的 url request

用來產生 POST request 的 NSMutableURLRequest Category

+(instancetype)PIXURLRequestForOauth2POST:(NSString *)urlString parameters:(NSDictionary *)parameters{
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:urlString]];
    request.HTTPMethod = @"POST";
    //設定 http header
    NSString *contentTypeValue = [NSString stringWithFormat:@"multipart/form-data; charset=utf-8; boundary=%@", kBoundaryConstant];
    [request addValue:contentTypeValue forHTTPHeaderField:@"Content-Type"];
    //設定 http body
    [request setHTTPBody:[stringForFormData(parameters) dataUsingEncoding:NSUTF8StringEncoding]];
    return request;
}
static NSString* stringForFormData(NSDictionary *parameters){
    // 做為 form data 參數間的分隔線，前面要有兩個減號
    NSString *boundaryWtihNewLine = [NSString stringWithFormat:@"--%@%@", kBoundaryConstant, kNewLineSymbol];
    __block NSMutableArray *array = [NSMutableArray array];
    [parameters enumerateKeysAndObjectsUsingBlock:^(NSString  *_Nonnull key, NSString  *_Nonnull obj, BOOL * _Nonnull stop) {
        NSString *string = [NSString stringWithFormat:@"Content-Disposition: form-data; name=\"%@\"%@%@%@%@", key, kNewLineSymbol, kNewLineSymbol, obj, kNewLineSymbol];;
        [array addObject:string];
    }];
    NSString *string = [array componentsJoinedByString:boundaryWtihNewLine];
    // 做為 form data 所有參數的結尾的 boundary，前後都要有兩個減號
    string = [NSString stringWithFormat:@"%@%@--%@--", boundaryWtihNewLine, string, kBoundaryConstant];
    return string;
}