Fixed a bug caused by generic registration of the enum class

but it could catch the user unaware for any registered non-leaf
class.  When a class has a field of a specific final type then
the type information is implicit in the outer object... and
the proper serialization information can be written.  It is then
sometimes useful to register a generic serializer for something
like Enum to catch these cases.  However, there are many times where
the type cannot be implicitly known and so must be specifically
registered.  Prior to this fix, having a generic Enum serializer
registered would cause problems on the reading end because it
wouldn't know what real enum class to use.
Now we catch the issue on the write side where enough information
is known to properly report this to the user.  Only a few Bothans
died to bring this info.

jme3-networking/src/main/java/com/jme3/network/serializing/Serializer.java

@@ -425,6 +425,22 @@ public abstract class Serializer {
             return;
         }
         SerializerRegistration reg = writeClass(buffer, object.getClass());
+        
+        // If the caller (or us) has registered a generic base class (like Enum)
+        // that is meant to steer automatic resolution for things like FieldSerializer
+        // that have final classes in fields... then there are cases where the exact 
+        // type isn't known by the outer class.  (Think of a message object
+        // that has an Object field but tries to send an Enum subclass in it.)
+        // In that case, the SerializerRegistration object we get back isn't
+        // really going to be capable of recreating the object on the other
+        // end because it won't know what class to use.  This only comes up
+        // in writeclassAndObejct() because we just wrote an ID to a more generic
+        // class than will be readable on the other end.  The check is simple, though.
+        if( reg.getType() != object.getClass() ) {
+            throw new IllegalArgumentException("Class has not been registered:" 
+                    + object.getClass() + " but resolved to generic serializer for:" + reg.getType());
+        } 
+
         reg.getSerializer().writeObject(buffer, object);
     }