NegotiationAuthenticator loses post data

The NegotiationAuthenticator loses post data.

A customer is attempting to use Negotiation along with PicketLink at the IDP. This works fine as long as the SP is using HTTP-Redirect SAML binding.

If the SP is using HTTP-Redirect, then this issue is avoided as the SAMLRequest is passed along through the redirects on the URL.

If the HTTP-POST binding is used, then the NegotiationAuthenticator will lose the SAMLRequest post parameter. This means that after a user is successfully authenticated, the IDP will not know where to redirect the user to. As a result, the user will be left at the IDP index.html page.