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/IPv4Address.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4Address.java
index 73b1a7d..43a7a17 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4Address.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv4Address.java
@@ -1,5 +1,7 @@
 package org.projectfloodlight.openflow.types;
 
+import java.util.Arrays;
+
 import org.jboss.netty.buffer.ChannelBuffer;
 
 import com.google.common.hash.PrimitiveSink;
@@ -133,7 +135,7 @@
                 }
             }
         }
-        return bytesCache.clone();
+        return Arrays.copyOf(bytesCache, bytesCache.length);
     }
 
     @Override