Fix AlphaNumericMessage.equals

I noticed excessive CPU usage in HashMap lookups in StatsDAggregator.
The stacks shows that the HashMap had devolved into a TreeMap which
happens after frequent hash collisions. That got me looking at the
implementations of hashCode/equals in the Message heirarchy and
this seems a likely culprit.

src/main/java/com/timgroup/statsd/AlphaNumericMessage.java

@@ -55,7 +55,7 @@ public boolean equals(Object object) {
 
         if (object instanceof AlphaNumericMessage ) {
             AlphaNumericMessage msg = (AlphaNumericMessage)object;
-            return super.equals(msg) && (this.value == msg.getValue());
+            return super.equals(msg) && (this.value.equals(msg.getValue()));
         }
 
         return false;