Corrected a regression in validation of admin fieldsets with nested...

                raise ImproperlyConfigured("'fields' key is required in "

                        "%s.fieldsets[%d][1] field options dict."

                        % (cls.__name__, idx))

            for field in fieldset[1]['fields']:

            for fields in fieldset[1]['fields']:

                # The entry in fields might be a tuple. If it is a standalone

                # field, make it into a tuple to make processing easier.

                if type(fields) != tuple:

                    fields = (fields,)

                for field in fields:

                    check_formfield(cls, model, opts, "fieldsets[%d][1]['fields']" % idx, field)

                    try:

                        f = opts.get_field(field)

class Book(models.Model):

    name = models.CharField(max_length=100)

    subtitle = models.CharField(max_length=100)

    price = models.FloatField()

    authors = models.ManyToManyField(Author, through='AuthorsBooks')

   ...

ImproperlyConfigured: 'FieldsetBookAdmin.fieldsets[1][1]['fields']' can't include the ManyToManyField field 'authors' because 'authors' manually specifies a 'through' model.

>>> class NestedFieldsetAdmin(admin.ModelAdmin):

...    fieldsets = (

...        ('Main', {'fields': ('price', ('name', 'subtitle'))}),

...    )

>>> validate(NestedFieldsetAdmin, Book)

# Regression test for #12209 -- If the explicitly provided through model

# is specified as a string, the admin should still be able use

# Model.m2m_field.through