Fix setting the 'Class' attribute on Accounts.

If you try to set the 'Class' attribute on an account, you'll get a
NoMethodError about the method "class=" not existing. This patch will try
the straight key name first, and if that fails, fall back to the field's
internal name ('account_class', in this case).
Allow payment dates in string format.
Merge branch 'fix-account-class-error' into fix-credit-notes-method
Merge branch 'allow-string-payment-dates' into fix-credit-notes-method