WireTap EIP - new message mode - Allow to specify multiple expression when creating the new message

See DEV-2882

Currently you can only use one expression to set a new message body when wire tapping a new message.
If you want to set a new body and a new header then you cannot do that easily.

The goal is to make it easy in the XML DSL as well so you can set headers there also. It should be possible to set 1+ headers.