Format Money Values with Trailing 0 Characters

[calebkleveter]

Format money decimal values with trailing 0 characters, so there are as many fraction digits as the currency exponent number:

85.8 USD => 85.80
10 USD => 10.00
10 XXX => 10

While most PayPal endpoints don't require this padding, some do, so we just do it for all models because it is easier that way and shouldn't break anything.