MULE-6871: DefaultMuleSession got broken between mule 3.2 and 3.3

_ Problem was originated adding transient modifier to the properties field.
_ Fxing the problem removing the transient modifier and updating the writeObject method.

This change will break interoperability between old 3.3.x/3.4.0 with 3.4.1 and newer versions. That old versions need to be patched to adapt to this change if required
(cherry picked from commit a25d793)