Export crashes if the column name is used instead of the attribute name

[matthewhegarty]

Describe the bug

Export will crash if you use a column_name in the fields list. v3 works fine if you use either the column_name or attribute.

To Reproduce

Using EBook, change the meta fields list to

fields = ("id", "author_email", "name", "published_date")

When exporting, the process will crash with KeyError

Versions (please complete the following information):

• Django Import Export: latest

Expected behavior

Either a decent error message, or handle the use of the column name. (import handles the use of the column name)