Possible regression away from express API

[jimisaacs]

So before this commit: b3fede1

an API consumer was passing an array of functions as base without a problem. As this documentation shows should be allowed, if this is a drop-in replacement for express: https://expressjs.com/en/api.html#app.use

A string representing a path.
A path pattern.
A regular expression pattern to match paths.
An array of combinations of any of the above.

Though checking for function breaks that contract, and is breaking the API consumer I mentioned after this change.