change order of fn name resolution

This is to have the same order as koa/lib/application.js:

debug('use %s', fn._name || fn.name || '-');
cf https://github.com/koajs/koa/blob/master/lib/application.js#L83

what is the rationale for you between the 2 version ?

I am working on an "instrumentation" middleware and am trying to see how I can "rename" anonymous or non anonymous middleware so that debug / audit traces are optimally readable.

I don't know why fn._name was there in the first place so I thought it was there for renaming purpose.

I would even prefer something like

    name = fn._rename || fn.name || fn._name_default || "-"

so that you can :
 - force a name (_rename)
 - add a default name when fn is anonymous

both case are useful depending on the instrumentation. I largely prefer the "rename" case fn._name || fn.name