Long book titles appear in mPDF running headers, even when a short title is provided

[josieg]

Description

When a book has a long title, the full title appears in the mPDF running header rather than being replaced by the short title.

Expected behaviour

I expect that when I have a long book title and I have set a short title on the Book Info page, that the short title will be used in the mPDF running header.

Actual behaviour

Currently, the full book title will appear in the mPDF running head, even when the book title is very long and a short title has been provided.

Steps to reproduce the problem

1. Create a book with a long title.

2. Set a short title on the Book INfo page.
   

3. Export book as mPDF.

4. Look at the running head. You should see the book's full title.