`Export to HTML with useful anchors` breaks LaTeX export

Hi,

I love `Export to HTML with useful anchors` for my HTML export. However, I just found that it breaks LaTeX export - it seems it forces the same label in LaTeX export. Then the .tex file ends up with labels like `\label{Section%20Header}`, and pdflatex complains about `%20` part.

I looked into the code a little bit to figure out a way to turn this off for LaTeX export, but have not been able to. It would be great if you can add a conditional where it does not modify anchors for LaTeX export.

Thank you again for the useful code!