title: Change values to lowercase before importing to Django
body: <p>I am using Django Import-Export to import data from an excel(xlsx) file to my Django Models.
  What I have works but I want it to consider strings with characters that have different cases to be the same. Currently if I have 'Black Sabbath' and 'black sabbath' in a column, the import considers them to be different artists. </p>
  
  <p>Is it possible to change all string values to lowercase during the import process? Or somehow configure Django to consider them the same?</p>
  
  <p>resources.py</p>
  
  <pre><code>class ArtistResource(resources.ModelResource):
  
      class Meta:
          model = Artist
          import_id_fields = ('artist',)
          skip_unchanged = True
  </code></pre>
  
  <p>models.py</p>
  
  <pre><code>class Artist(models.Model):
      artist = models.CharField(primary_key=True, unique=True, max_length=30)
  
      def __str__(self):
          return self.artist
  </code></pre>
  
  <p>admin.py</p>
  
  <pre><code>class ArtistAdmin(ImportExportActionModelAdmin):
      resource_class = ArtistResource
  </code></pre>