officially support mutating middleware

Because of #65, specifically lodash.flatten, compose() returns a generator based on a copy of the provided array from that point in time. Any changes made to this array subsequently will no longer be reflected in the returned middleware.

Previously, one could mutate the array after calling compose.

[conorhastings]

@jonathanong The problem occurs if you call app.use to add any middleware after you’ve already triggered a call of app.callback, since this.middleware, the argument that is passed to compose is a mutable array.

this would previously work fine as it would reference the now mutated array, but since the flatten changes the semantics by copying at call time of compose with the flatten, anything added to the middleware after the call to compose is effectively ignored.

I would think that even if this wasn’t an envisioned use case, it still seems like a breaking change since it does, even if inadvertently, change the way the module works.

Thanks for all the hard work!

[jonathanong]

instead of supporting it here, we will add an ability to configure your compose function in koa https://github.com/koajs/koa/issues/1568