Add new parameter mandate_reference

[stoilkov]

A new optional parameter is available for both Transaction and Subscriptions
"mandate_reference" - non empty string.

For transactions:

Update the Transaction model ( add mandateReference String)
Update the create method(s) (as optional parameter)

For Subscriptions

Update the Subscription model ( add mandateReference String)
Update the create method(s) (as optional parameter)
Update the update method (make Subscrition.mandateReference @Updateable )

Template: paymill/paymill-php@51800d6

[sowinski]

Why is this still not implemented? Can someone fix this please? :) Its almost 2 years ago