send fails every other run.

Appologize for the gif. However a gif is worth 1000 words. 😎

[The following code is running from snuggsi ツ](https://github.com/devpunks/snuggsi/blob/47af2f96ccc7ab7a0883c2e1a33fecbc24746268/mixins/middleware.es):
```javascript
const
root = 'mixins'
, mime = /^\*\/\*$/
, filter = /^\/mixins\/(.*\.es)*$/g

, send = require ('koa-send')

module.exports = async (context, next) => {

const
match = filter.test (context.path)
, resource = './storable.es'

, settings = [context, resource, { root }]

, header = ['Content-Type', 'application/ecmascript']
, response = match && await send ( ... settings )

void (response && context.set ( ... header ))
|| await next ()

console.log ('response', context.response, response)
}
```

By the way it's a little wonky to change headers AFTER send is called. (which just returns a file path anyways). Let me know where I can help. PR. etc.

Thanks in advance @tj

Reproduce the alternating failure using the middleware example in mixins/middleware.es, then inspect the filter.test(context.path) and koa-send interaction. Confirm the expected behavior for matching paths, file responses, and response headers, and add a regression test showing that repeated requests behave consistently.

Written by the indexing model from the issue text.