Fix ManyToMany field default filter type

[weilu]

It's a bug introduced in this commit: 2d4ca0a (part of PR #1119) It manifests as not accepting a list of IDs [ID] but rather a single ID ID as filtering arguments for a m2m field. The bug can be replicated by adding a many-to-many relationship in the cookbook example app. Happy to clean up what I have locally and push it to a branch for quick reference if necessary.

I do want to add test cases to this PR but I thought it's best/fastest to have @zbyte64 or @tcleonard point me to where's the best place to add it as they are more familiar with the existing fixtures & setup. Happy to add on.

Also, side question to @tcleonard on his modification of the function get_filtering_args_from_filterset in the above-mentioned commit, why adding all the type checking branching code in this method itself rather than expanding/modifying model_field.formfield as based on my quick read that seems to be where the logic of translating model_field to form_field is intended to live?

[keithhackbarth]

@weilu - Great work. As you called out already, it would be helpful to see a breaking test here as an example. You can see the existing tests in filter/tests and I think you can use that format as a guide to how to write one for this.