Item mass update form is ignoring JMX endpoint.

Steps to reproduce:

1. Create an item (any will do, but not JMX agent type)
2. Open mass update form for this item and try changing item type to JMX agent and JMX endpoint to any string (I used "HEY! ENDPOINT IS HERE!!!" as an example).
3. Update an item.
4. Observe that JMX endpoint value is ignored and error message is produced:

   Incorrect value for field "jmx_endpoint": cannot be empty.

Thanks to gcalenko for pointing this out.