paynow 2.1.2

Paynow Client helper package.

[1.0.7] - Fix Paynow Redirect Error

[1.1.0] - Patch Payment Stream Listener

[2.0.0] - Enable null-safety

[2.1.1] - Fix null-safety issues

[2.1.2] - Change amount type to String [obeying paynow responses]