Return the middleware's result so that async error handling works.

[N-McA]

By returning the result of the middleware, promise chaining can be done elsewhere. An example of why this is (very) useful is given by

https://github.com/N-McA/express-ws-async-error-fail-example

Which shows the lack of interoperability that occurs without the suggested change.

[anistark]

@N-McA On node 12.5.0 I'm getting error TypeError: middleware is not a function.
Adding return didn't help.