java_gen: use Arrays.copyOf() instead of .clone() for defensive copy

@gregor-bsn convinced me it's better :)
diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/MacAddress.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/MacAddress.java
index 48a4faf..f592ea8 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/MacAddress.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/MacAddress.java
@@ -1,5 +1,7 @@
 package org.projectfloodlight.openflow.types;
 
+import java.util.Arrays;
+
 import org.jboss.netty.buffer.ChannelBuffer;
 import org.projectfloodlight.openflow.exceptions.OFParseError;
 import org.projectfloodlight.openflow.util.HexString;
@@ -95,7 +97,7 @@
                 }
             }
         }
-        return bytesCache.clone();
+        return Arrays.copyOf(bytesCache, bytesCache.length);
     }
 
     /**