Ensure redirect_to links produce an HTML file.

[parkr]

Debugging @gboone's 18f.gsa.gov site, I noticed this:

$ tree _site/tags/how\ we\ work
_site/tags/how\ we\ work/
├── index
├── index.html

I had seen something like this before due to #96. The problem is a naïve File.basename(permalink), which takes how we work from /tags/how we work/ instead of /. By appending an index.html, we ensure that the redirect page has the proper basename to be properly created.

Fixes #107.

[benbalter]

LGTM, pending the merge conflict.

[parkr]

@benbalter: rebased.

[benbalter]

@jekyllbot: merge