[BugFix] Fix OAuth login handshake - final phase

The OAuth login handshake was seriously broken: when it
was completed the final /login did not redirected back to 
Gerrit.

Now it is fully working again, as if we are already logged in
we just enrich the request and proceed to the normal
Gerrit filter flow.

Change-Id: I49ffa752a1684a14711a9356b2437be38b4d2844

github-oauth/src/main/java/com/googlesource/gerrit/plugins/github/oauth/OAuthWebFilter.java

diff --git a/github-oauth/src/main/java/com/googlesource/gerrit/plugins/github/oauth/OAuthWebFilter.java b/github-oauth/src/main/java/com/googlesource/gerrit/plugins/github/oauth/OAuthWebFilter.java
index 2b0b774..ebd31bc 100644
--- a/github-oauth/src/main/java/com/googlesource/gerrit/plugins/github/oauth/OAuthWebFilter.java
+++ b/github-oauth/src/main/java/com/googlesource/gerrit/plugins/github/oauth/OAuthWebFilter.java
@@ -87,7 +87,8 @@
 
       if (OAuthProtocol.isOAuthLogout(httpRequest)) {
         logout(request, response, chain, httpRequest);
-      } else if (OAuthProtocol.isOAuthRequest(httpRequest)) {
+      } else if (OAuthProtocol.isOAuthRequest(httpRequest)
+          && !ghLogin.isLoggedIn()) {
         login(request, httpRequest, httpResponse, ghLogin);
       } else {
         httpRequest = enrichAuthenticatedRequest(httpRequest, authCookie);