/// <summary>
/// 添加字段到已有的Content
/// 要求content-type为application/x-www-form-urlencoded
/// </summary>
/// <param name="keyValues">键值对</param>
/// <exception cref="NotSupportedException"></exception>
/// <returns></returns>
public async Task AddFormFieldAsync(IEnumerable <KeyValue> keyValues)
{
this.EnsureMediaTypeEqual(FormContent.MediaType);
var formContent = await FormContent.ParseAsync(this.Content).ConfigureAwait(false);
formContent.AddFormField(keyValues);
this.Content = formContent;
}
