ReadOnly RichTextField still accepts new lines

Steps to reproduce

1. Create a dialog that incorporates a richTextField where readOnly = true.
2. Open said dialog and attempt to edit the field. You cannot enter any characters and any preset information is uneditable but new lines can be inserted.
3. (See attached zip archive for an example content app with a 'preview' action)

Expected results

1. As the field is uneditable, it should not allow users to think they are making changes.

Actual results

1. The user can insert new lines in the body of text.
2. Note: On using a save action, the new lines are not persisted.

Development notes

1. This does not appear in the CKEditor 4 issues list. See: https://github.com/ckeditor/ckeditor4/issues?page=1&q=is%3Aissue+readonly