title: Cannot Signout the External Identity provider in IdentityServer
body: <p>I've an MVC Application which uses IdentityServer4. In IdentityServer4, I registered SAML2 (SustainSys.SAML2) as the external Login provider. and Login works fine. </p>
  
  <p>When user log out of the MVC application, it logs out from the MVC application but the log out for External Login Provider isn't triggering. I checked the LogOut method of my identity Server which does the redirect to External Authentication Scheme. but the redirect doesnt happen. </p>
  
  <pre><code>  this triggers a redirect to the external provider for sign-out
      return SignOut(new AuthenticationProperties { RedirectUri = url }, 
      vm.ExternalAuthenticationScheme);
  </code></pre>
  
  <p>And here is the code where in i registered External Identity Provider for SAML. I've used Nuget package from SustainSys SAML. </p>
  
  <pre><code>.AddSaml2(options =&gt;
  {
      options.SignInScheme = IdentityServerConstants.ExternalCookieAuthenticationScheme;
      options.SignOutScheme = IdentityServerConstants.SignoutScheme;
      options.SPOptions = CreateSPOptions();
      var idp = new IdentityProvider(new EntityId(_strIDPEntityId), options.SPOptions)
      {
          AllowUnsolicitedAuthnResponse = true,
          Binding = Saml2BindingType.HttpRedirect,
          SingleSignOnServiceUrl = new Uri(_strSingleSignOnURL),
          SingleLogoutServiceBinding = Saml2BindingType.HttpRedirect,
          SingleLogoutServiceUrl = new Uri("https://devit-dev.onelogin.com/trust/saml2/http-redirect/slo/1111")
  
      };
      idp.SigningKeys.AddConfiguredKey(
      new X509Certificate2(
          AppDomain.CurrentDomain.SetupInformation.ApplicationBase + "../../../App_Data/OneLogin.cer"));
      options.IdentityProviders.Add(idp);
  });   
  </code></pre>
  
  <p>Not sure what am i missing here. Appreciate any help.</p>