It looks like sometimes, particularly when the first thing we do after
loading a page is filling the CKEditor fields and submitting the form,
CKEditor doesn't have enough time to format the text, and so it's sent
as plain text instead of HTML. This behaviour can be reproduced on my
local machine after upgrading to Rails 5.1, with the test "Admin Active
polls Add" failing 100% of the time.
Checking CKEditor has been filled in correctly solves the issue.
2cdc6a1b1b