Rare issue with ttfunk library

[nickca...@gocardless.com]

Hi,

We've encountered a couple of exceptions in production with prawnpdf/ttfunk, which raises a TypeError (no implicit conversion of false into Integer).

There is an outstanding pull request to fix this (it's quite simple: https://github.com/prawnpdf/ttfunk/pull/40) - would one of the maintainers mind taking a look please? Happy to contribute if needed (tests, reviews, etc).

Thanks in advance! Nick