Redirect polygerrit urls with context to GWTUI

When CanonicalWebURL contains a context like 'gerrit'.
https://a-host/gerrit the context was not taken into consideration
when redirecting polygerrit urls to GWT urls.

pg-uri: /gerrit/c/123/
context + '#' +  pg-uri = /gerrit/#/gerrit/c/123

Strip context from pg-uri before transforming it to GWT url.

Change-Id: I7aa10c53a40c21c3240f227d015827ca38e19da8

gerrit-httpd/src/main/java/com/google/gerrit/httpd/UrlModule.java

@@ -137,7 +137,7 @@ class UrlModule extends ServletModule {
           @Override
           protected void doGet(final HttpServletRequest req, final HttpServletResponse rsp)
               throws IOException {
-            toGerrit(req.getRequestURI(), req, rsp);
+            toGerrit(req.getRequestURI().substring(req.getContextPath().length()), req, rsp);
           }
         });
   }