Mantle 自动生成JSONKeyPathsByPropertyKey 的NSDictionary

• mantle中自带的mtl_identityPropertyMapWithModel 函数，可以做转换，用法

+(NSDictionary *)JSONKeyPathsByPropertyKey
{
    NSMutableDictionary *mapping = [[NSDictionary mtl_identityPropertyMapWithModel:self] mutableCopy];
    return mapping;
}