Fixed #22210 -- Saving model instances to non-related fields.

Previously, saving a model instance to a non-related field (in
particular a FloatField) would silently convert the model to an Integer
(the pk) and save it. This is undesirable behaviour, and likely to cause
confusion so the validatio has been hardened.

Thanks to @PirosB3 for the patch and @jarshwah for the review.

tests/model_fields/models.py

@@ -48,6 +48,9 @@ class Whiz(models.Model):
 class BigD(models.Model):
     d = models.DecimalField(max_digits=38, decimal_places=30)
 
+class FloatModel(models.Model):
+    size = models.FloatField()
+
 
 class BigS(models.Model):
     s = models.SlugField(max_length=255)