public OAuth2HeaderHandler(TumblrOAuth2Credentials tumblrOAuth2Credentials, HttpMessageHandler handler) : base(handler)
 {
     CredentialsHolder = tumblrOAuth2Credentials;
 }
 public OAuth2HeaderHandler(TumblrOAuth2Credentials tumblrOAuth2Credentials) : base(new HttpClientHandler())
 {
     CredentialsHolder = tumblrOAuth2Credentials;
 }