title: After changing field to read only, the form displays pk instead of value/ django forms
body: <p>When I add this line of code to my form:</p>
  
  <pre><code> borrower = forms.CharField(
      widget=forms.TextInput(attrs={'readonly': 'readonly'}))
  </code></pre>
  
  <p>The value of borrower is showing up as pk(id) instead of its real value. Any idea why is that?
  The complete code for form:</p>
  
  <pre><code>class AmendLoan(ModelForm):
    borrower = forms.CharField(
        widget=forms.TextInput(attrs={'readonly': 'readonly'}))
  
    class Meta:
        model = BikeInstance
  
        fields = (
            'borrower',
            'frame_number',
            'due_back',
            'status'
        )
  </code></pre>
  
  <p>When I remove <code>readonly</code>, everything works fine, but I need this field to show up, but it cant be changed.</p>