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).
Merge pull request #145 from fimmtiu/fix-account-class-error

Fix Setting the 'Class' Attribute on Accounts.