Persist original Meta class on Options instances

[Tim McCurrach]

When a model.Model class is created ​the metaclass `pop`s off any Meta attribue defined on the model ​before calling super.__new__(). We do ​pass it as an argument to the Options class, and ​it's stored on the Options instance, however ​this is then deleted whilst the Options.contribute_to_class class is called.

As far as I can tell, there is no reason why it is important to delete the original data stored in the class defined on the original model. Admitedly, perhaps it could be stored with a better name to indicate it is a non-functional reference to the original Meta class. instance._meta.meta probably doesn't make much sense, but instance._meta.original_meta might be more appropriate.

Motivation for feature request

The motivation for this came change is born out of a desire to write a check that db_table is explicitly defined on all my models (even if it matches the default value that django would apply). I can write checks that require things like ordering is defined (or not defined) on a model. The problem with db_table is that defaults are applied when contribute_to_class is called. This means we need a reference to the underlying original Meta data. This is not currently possible without subclassing the metaclass (ModelBase) which is a step further than would be ideal imo.

The fix

The fix should be very simple. We would just need to remove (or adapt) the line of code linked above which deletes the meta-class. (We could maybe use a better name than original_meta!!)