user.Info.Extra keys can have '/' in them ( we actually advertise this in the doc) but header keys cannot. This is problematic because we transport those keys embeded in header keys when we use the requestheader authenticator. We have a couple options that I can think of. We can either:

1. Encode extra info differently.
2. Tighten extra info key validation.

Go validation here:
https://github.com/golang/go/blob/1c69384da4fb4a1323e011941c101189247fea67/src/net/textproto/reader.go#L593-L602

@yliaog @seans3 @deads2k @liggitt