Fix to_pem in new Ruby versions (merge pull request)

[collimarco]

The to_pem method is broken in all the new Ruby versions due to upstream changes in the OpenSSL library.

@zaru @rossta @mohamedhafez can someone please merge my pull request in order to fix the issue? It also improves test coverage for PEM.

Here's the pull request: https://github.com/zaru/webpush/pull/97

Thanks

[collimarco]

We have been using the new code (pull request) in production for a month and it works perfectly...

@zaru Can you please merge when you have time?