title: Adding dynamic fields to Django's export_as_csv action in admin
body: <p>I'm trying to customize the <code>export_as_csv</code> action that appears in the admin menu, so that I can add dynamic fields to the exported CSV file. This is through the use of <a href="https://django-adminactions.readthedocs.org/en/latest/" rel="nofollow">django-adminactions</a> which provides the <a href="https://django-adminactions.readthedocs.org/en/latest/actions.html#export-as-csv" rel="nofollow">Export as CSV</a> action. So far I have the following:</p>
  
  <pre><code>from django.contrib import admin
  from adminactions.api import csv_options_default, export_as_csv as _export_as_csv
  from myapp.models import MyModel
  
  class MyModelAdmin(admin.ModelAdmin):    
      list_display = ('field1', 'field2', 'field3')
  
      qs = MyModel.objects.all()
      options = csv_options_default
      fields = ['field4', 'field5']
      header = ['Header4', 'Header5']
      actions = [_export_as_csv(queryset=qs, fields=fields, header=header, options=options)]
  
  admin.site.register(MyModel, MyModelAdmin)
  </code></pre>
  
  <p>But the above generates an error on the page as follows:</p>
  
  <pre><code>hasattr(): attribute name must be string
      Request Method: GET
      Request URL:    http://localhost:8000/admin/mymodel/mymodel/
      Django Version: 1.4.14
      Exception Type: TypeError
      Exception Value:    
      hasattr(): attribute name must be string
      Exception Location: /home/me/.virtualenvs/myproj/local/lib/python2.7/site-packages/django/contrib/admin/options.py in get_action, line 648
      Python Executable:  /home/me/.virtualenvs/myproj/bin/python
      Python Version: 2.7.6
  </code></pre>
  
  <p>I'm I doing this the right way? How would one go about adding dynamic fields to Django's <code>export_as_csv</code> function in the admin?</p>