Description of the problem, including code/CLI snippet

Breaking out discussion fromthis Pull Request thread into its own issue.

The Save and Update mixins forprotectedbranch doesn't really work for most of it's attributes, as the PATCH/POST api use different values from those return by the API.

The PATCH api requires to sendallowed_to_push attribute, but the returned attribute ispush_access_levels, as documented in the API and examples:https://docs.gitlab.com/ee/api/protected_branches.html#example-update-a-push_access_level-record

Expected Behavior

To be able to update the `allowed_to_push' GitLab protected branch state.

Actual Behavior

Using the python-gitlab library to modify thepush_access_levels attribute that the object has and then calling.save() appears to do nothing, and the python object has noallowed_to_push attribute to modify.

It does work for the basic.allow_force_push protectedbranch attribute, as added and tested in the originalPull Request.

Specifications

• python-gitlab version: unreleased (a867c48)
• API version you are using (v3/v4):v4
• Gitlab server version (or gitlab.com):gitlab.com