stdlib log.SetOutput does not work with the *Middleware's

[thomasf]

Suggested change: Default to make a function call to log.Print instead of creating new logger if *AccessLogMiddleware.Logger is nil. It seems like it's a very uncommon pattern to create new loggers if there isn't a very specific reason for it.

[ant0ine]

That would be an API change, and that won't happen before the next major version for compatibility reason. Filed under "v4 idea"