Add support for the prompt parameter in Salesforce

Owin.Security.Providers/Salesforce/SalesforceAuthenticationHandler.cs

@@ -198,6 +198,11 @@ namespace Owin.Security.Providers.Salesforce
                     ""
                     );
 
+                if (Options.Prompt != null)
+                {
+                    authorizationEndpoint += string.Format("&prompt={0}", Options.Prompt);
+                }
+
                 Response.Redirect(authorizationEndpoint);
             }
 

Owin.Security.Providers/Salesforce/SalesforceAuthenticationOptions.cs

@@ -94,6 +94,15 @@ namespace Owin.Security.Providers.Salesforce
         /// </summary>
         public IList<string> Scope { get; private set; }
 
+        /// <summary>
+        /// Specifies how the authorization server prompts the user for reauthentication and reapproval. This parameter is optional. 
+        /// The only values Salesforce supports are:
+        /// login—The authorization server must prompt the user for reauthentication, forcing the user to log in again.
+        /// consent—The authorization server must prompt the user for reapproval before returning information to the client.
+        /// It is valid to pass both values, separated by a space, to require the user to both log in and reauthorize. 
+        /// </summary>
+        public string Prompt { get; set; }
+
         /// <summary>
         ///     Gets or sets the name of another authentication middleware which will be responsible for actually issuing a user
         ///     <see cref="System.Security.Claims.ClaimsIdentity" />.