Break out setupRedfishRoute

Previously, we had a pattern where we would call the HEAD handler for a
route in the GET handler.  Unfortunately, this leads to the error from
setupRedfishRoute to not be handled, which means that for these routes,
400 isn't processed properly.

Fix the code.

Tested: Redfish service validator passes.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I324cc9582f2368a08265e1defee5b6c56ded3267
diff --git a/redfish-core/lib/redfish_sessions.hpp b/redfish-core/lib/redfish_sessions.hpp
index 065878e..494f15c 100644
--- a/redfish-core/lib/redfish_sessions.hpp
+++ b/redfish-core/lib/redfish_sessions.hpp
@@ -64,7 +64,13 @@
                      const std::shared_ptr<bmcweb::AsyncResp>& asyncResp,
                      const std::string& sessionId)
 {
-    handleSessionHead(app, req, asyncResp, sessionId);
+    if (!redfish::setUpRedfishRoute(app, req, asyncResp))
+    {
+        return;
+    }
+    asyncResp->res.addHeader(
+        boost::beast::http::field::link,
+        "</redfish/v1/JsonSchemas/Session/Session.json>; rel=describedby");
 
     // Note that control also reaches here via doPost and doDelete.
     auto session =