Contribution import parser gets custom fields for Contact

The Contribution import parser allows custom fields to be imported. As it reads in values, it reformats Dates and Booleans so they are palatable to CiviCRM.

A call is made to CRM_Core_BAO_CustomField::getFields() however, the code was copied incorrectly from the Import Contact parser, and asks for custom fields for Contacts, not Contributions. This is a one-line change, and I'll shortly supply a PR.