caddyhttp: Fix common_log format's user ID placeholder

[francislavoie]

I was digging around for the http.auth.user.id placeholder in the code and noticed that the common_log format was using the wrong placeholder.

[francislavoie]

As an aside, I don't think {http.auth.user.id} is documented anywhere. That should probably be added in a couple different spots (basicauth directive and in the authentication module docs I figure)

[mholt]

Thanks for the fix!

You are right that it's not documented anywhere -- yet; this is intentional as the auth features aren't really finished/set in stone yet, but there's some good progress on auth plugins coming along by the community so I think we can document them better soon enough!