Replace Funnels.stringFunnel with Funnels.unencodedCharsFunnel

In Guava 15.0 the `stringFunnel()` method is deprecated in favor
of `unencodedCharsFunnel()`.

Change-Id: Ie99e3305f5ec497a36033142283dd5994803a254

gerrit-cache-h2/src/main/java/com/google/gerrit/server/cache/h2/H2CacheImpl.java

@@ -321,7 +321,7 @@ public class H2CacheImpl<K, V> extends AbstractLoadingCache<K, V> {
       @SuppressWarnings("unchecked")
       @Override
       Funnel<String> funnel() {
-        Funnel<?> s = Funnels.stringFunnel();
+        Funnel<?> s = Funnels.unencodedCharsFunnel();
         return (Funnel<String>) s;
       }
     };