SPARK-12186: Add a comment explaining the changes. Convert it to a one-liner.

Author: Rohit Agarwal

core/src/main/scala/org/apache/spark/deploy/history/HistoryServer.scala

@@ -103,11 +103,9 @@ class HistoryServer(
       // Note we don't use the UI retrieved from the cache; the cache loader above will register
       // the app's UI, and all we need to do is redirect the user to the same URI that was
       // requested, and the proper data should be served at that point.
-      if (req.getQueryString == null) {
-        res.sendRedirect(res.encodeRedirectURL(req.getRequestURI()))
-      } else {
-        res.sendRedirect(res.encodeRedirectURL(req.getRequestURI + "?" + req.getQueryString))
-      }
+      // Also, make sure that the redirect url contains the query string present in the request.
+      val requestURI = req.getRequestURI + Option(req.getQueryString).map("?" + _).getOrElse("")
+      res.sendRedirect(res.encodeRedirectURL(requestURI))
     }
 
     // SPARK-5983 ensure TRACE is not supported