html_class_attr is not being cleared in loop within forms._html_output

[Skidd]

The _html_output() method within django.forms.BaseForm
has a for loop that creates a TR for each field.
The html_class_attr field is not being cleared while iterating through the loop.
As soon as one field has this value set, all following fields get it too.

I found this while using a ModelForm that was forcing all fields to required.

Fix:

    if css_classes:
        html_class_attr = ' class="%s"' % css_classes
    else: 
        html_class_attr = ''

or

    for name, field in self.fields.items():
        html_class_attr = ''

Thanks

[Jannis Leidel]

Duplicate of #12466