Add Client Certificate Options

[nikz]

Hi Folks,

We've been using something like the following:

https://github.com/xero-gateway/xero_gateway/blob/master/lib/oauth/oauth_consumer.rb

To add in support for SSL client certificates.

I'm happy to work that into a proper PR, just checking that's something that would be acceptable to y'all?

Thanks!

Nik

[mpapis]

Hey, sorry for slow answer, of course there will be someone to review :)

[nikz]

Perfect, I will work this up into a proper PR over the coming months.