cors: always return 204 for a matched preflight

[gregwebs]

Previously it could return a 404, etc. This had the effect of in the browser producting a network error because the preflight request failed.

These changes are passing the test suite but are not yet manually tested.

[gregwebs]

So this will still end up having the 404 issue because it specifically mounts to existing routes. But it will avoid other issues with a middleware returning a 401, etc.

[raphael]

Looks great, thank you!

[gregwebs]

I tested this locally and am now seeing 204 instead of 200 and it is working in the browser