fix: generate multipage TOC with http_prefix

[lhokktyn]

Please see replication steps here: https://github.com/alphagov/tech-docs-template/issues/183#issuecomment-478942982

Whilst the test already in place appears to work, I'm not sure it is covering my particular case because when I build my site it is not generating files within a prefixed folder, so I may need a bit of guidance to write additional test (or correct my approach!) ...

For example, when http_prefix=/prefix/, given my source/:

|- source/
|--documentation/
|--index.html.md.erb
|--page2.html.md.erb

This is generating files in the build directory as so:

|- build/
|--index.html
|--page2.html

But the test in place appears to expect generated assets to be:

|- build/
|--prefix/index.html
|--prefix/page2.html

The attached PR does fix the issue for me, however.