REST views: regression in 8.5.x: view with display using 'csv' format now returns a 406 response, need to add ?_format=csv to URL to make it work

Problem/Motivation

Tested a large Drupal 8 project on 8.5.0-alpha1 today and started getting test fails for views that had rest exports powered by CSV serialization
The views have URLs like /some/path/to/download/content.csv
They work if I change the URLs to /some/path/to/download/content.csv?_format=csv

Proposed resolution

Is this an issue in CSV serialization or a regression in core?

Remaining tasks

User interface changes

API changes

Data model changes