Stringify all values coming from site.github

[WasabiFan]

This should convert values to string equivalents when getting them out of site.github. See discussion in jekyll/github-metadata#22 for more information.

@parkr

[parkr]

This looks fine! To test, please add a test that sets up a non-string in @site.config['github']['url'] and ensures the stringified value is returned. https://github.com/jekyll/jekyll-redirect-from/blob/master/spec/jekyll_redirect_from/redirector_spec.rb

[WasabiFan]

Will-do.

[WasabiFan]

I just added that test. I defined a new class to use as the non-string -- if you'd rather I use a different class (instead of defining one) I can.

[parkr]

Looks great, thanks, @WasabiFan!