Can't switch to default font size

[Comandeer]

Are you reporting a feature request or a bug?

Bug

Provide detailed reproduction steps (if any)

This bug is introduced in #584.

1. Open editor with font plugin loaded.
2. Set editor content to <p>Hello world!</p><p>Hello again!</p>
3. Select all and apply "Courier New" font family.
4. Select the second paragraph and apply font size of 36. Your content after these changes should look like <p><span style="font-family:Courier New,Courier,monospace;">Hello world!</span></p><p><span style="font-size:36px;"><span style="font-family:Courier New,Courier,monospace;">Hello again!</span></span></p>
5. Select all and apply default font size.

Expected result

Font size in second paragraph is back to the initial value.

Actual result

Font size doesn't change.

[f1ames]

Similar to #1116 (not sure if it is the same cause).