title: python django-import-export import model data even if there are not data
body: <p>I'm using <code>django-import-export</code> package to import data from a csv file, basically following the getting started tutorial in <a href="https://django-import-export.readthedocs.io/en/latest/getting_started.html" rel="nofollow noreferrer">readthedocs</a></p>
  
  <p>I'm confused with a behavior of the package in a simple configuration.</p>
  
  <p>My model is like:</p>
  
  <pre><code>class Experiment(models.Model):
      title = models.CharField(max_length=255, blank=False)
      description = models.TextField(blank=False)
      research_project = models.ForeignKey(
         ResearchProject, null=False, blank=False
      )
  </code></pre>
  
  <p>I created a csv file to test import. The file <code>/tmp/experiment.csv</code> is simple:</p>
  
  <pre><code>h1,h1,research_project
  v1,v2,66
  </code></pre>
  
  <p>(Note that the first two columns of csv have headers that not correspond to any of the fields models)</p>
  
  <p>Then, I created a Resource:</p>
  
  <pre><code>class ExperimentResource(resources.ModelResource):
  
  class Meta:
      model = Experiment
      exclude = ('id', )
  
  def export(self, queryset=None, *args, **kwargs):
      queryset = Experiment.objects.filter(id=kwargs['id'])
      return super(ExperimentResource, self).export(queryset, *args, **kwargs)
  
  def get_instance(self, instance_loader, row):
      return False
  </code></pre>
  
  <p>Now, I can try to import <code>/tmp/experiment.csv</code> data, by calling:</p>
  
  <pre><code>dataset = Dataset().load(open('/tmp/experiment.csv').read())
  result = ExperimentResource().import_data(dataset)
  print(result.has_errors())  # for debug
  </code></pre>
  
  <p>Last line prints <code>False</code>, as data has effectively imported and a line has added to the database. But <code>title</code> and <code>description</code> was filled with an empty string, even if in <code>Experiment</code> model this two attributes have <code>blank=False</code>.</p>
  
  <p>My questions are: why <code>django-import-export</code> allows saving empty strings for models that have fields with <code>blank=False</code> parameters? And why the package do not check for the headers of the dataset for corresponding fields model?</p>
  
  <p>How to make <code>django-import-export</code> to check for headers and empty strings in fields that have <code>blank=False</code>?</p>