RestSession: Add helper method to send HEAD request to REST endpoint

Change-Id: I3ede7fa365e55157cba6732f1ed22d10f0a447c9

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/RestSession.java

@@ -54,6 +54,10 @@ public class RestSession extends HttpSession {
     return execute(get);
   }
 
+  public RestResponse head(String endPoint) throws IOException {
+    return execute(Request.Head(url + "/a" + endPoint));
+  }
+
   public RestResponse put(String endPoint) throws IOException {
     return put(endPoint, null);
   }