Allow non-standard http status codes to be used

Koa throws when a status code not defined in the statuses package is used in response:

https://github.com/koajs/koa/blob/78832ff6c63fdd93ad9a49f31ba2edd8cbe8adee/lib/response.js#L84

However, the HTTP RFC does not prevent custom status codes to be used: http://greenbytes.de/tech/webdav/draft-ietf-httpbis-p2-semantics-25.html#rfc.section.6.p.2

I can understand why status code validation might be useful for people just starting with http protocols, but for people who actually need to use custom codes, this is very limiting.

Proposal

There are several options that the Koa team could choose to remediate the problem:

• Do not throw an error and just emit a warning
• Keep throwing but add a configuration flag to disable the check (ie. koa.validateStatus = false)
• Allow adding custom http status codes to Koa so that it accepts them (ie. koa.AddStatus(440, 'Invalid Chicken'))

I personally like the second and third approach. Currently the third approach is possible, albeit in a hacky way:

import statuses from 'statuses'

// Add a hypothetical "invalid chicken" response code
statuses[440] = 'Invalid chicken'
statuses.codes.push(440)
statuses['Invalid chicken'] = 440

[fl0w]

Last option, which I would prefer, is not to expose an API for handling statuses but rather to expose the validation fn, which can be overloaded/replaced as needed.

[ruimarinho]

What about just validating that the status code is a number as per the HTTP spec?

The status-code element is a three-digit integer code giving the result of the attempt to understand and satisfy the request.