IdentityModel example is invalid (#2182)
IdentityModel example is invalid (#2182)

Current version of IdentityModel does not send client_id and client_secret when calling `RequestAuthorizationCodeAsync` (bug?), but IdentityServer states that client_id is required.
Working example:
`
var tokenClient = new TokenClient(d.TokenEndpoint, "client_id_xxx", "secret_yyy", style: AuthenticationStyle.PostValues);
 tokenClient.RequestAuthorizationCodeAsync("hdh922", "https://myapp.com/callback", extra: new {client_id = "client_id_xxx", client_secret = "secret_yyy"});
`
IdentityServer/IdentityServer4,MienDev/IdentityServer4,IdentityServer/IdentityServer4,MienDev/IdentityServer4,MienDev/IdentityServer4,IdentityServer/IdentityServer4,MienDev/IdentityServer4,IdentityServer/IdentityServer4