Allow non lowercase hashtags.

There is no reason hashtags need to be lowercase. Using
lowercase acronyms or issue tags looks really strange.

Note: The search is still case insensitive, as it should be.

Change-Id: I8a1f733deef92a6d0ddc6b6292d3d0a657246aba

gerrit-server/src/main/java/com/google/gerrit/server/change/HashtagsUtil.java

@@ -65,7 +65,7 @@ public class HashtagsUtil {
   public static String cleanupHashtag(String hashtag) {
     hashtag = LEADER.trimLeadingFrom(hashtag);
     hashtag = WHITESPACE.trimTrailingFrom(hashtag);
-    return hashtag.toLowerCase();
+    return hashtag;
   }
 
   private Set<String> extractTags(Set<String> input)