Handle the case where the access-control-request-header is not provided

[justinmchase]

This fixes issue #346

So for whatever reason the client I am using, when making an OPTIONS request to s3rver is not specifying any Access-Control-Request-Header headers at all. As a result s3rver erroneously responds with a 403 error. S3 proper does not have this same issue for the exact same request.

I have added a test which fails with vanilla master code and passes once I made the corresponding fix.

The issue appears to be here where if the header is not specified then requestHeaders ends up being [ '' ] rather than []. Then when we get to the check here the conditional expression allowedHeaders.length < requestHeaders.length fails because [].length < [ '' ].length === true.

Therefore my solution is to remove empty elements from the set since that should never happen provided there is a value specified in the header.

Before

curl:

$ curl -I -X OPTIONS -H "access-control-request-method: PUT" -H "origin: localhost:8888" "http://localhost:4569/example/abc123"
HTTP/1.1 403 Forbidden
Content-Type: application/xml; charset=utf-8
Content-Length: 344
ETag: W/"158-dYD/owyTT+IOwBQKGAb8GxwXdSU"
Date: Thu, 29 Nov 2018 21:23:53 GMT
Connection: keep-alive

s3rver:

$ node .
now listening on host 0.0.0.0 and port 4569
info: [S3rver] OPTIONS /example/abc123 403 344 - 3.963 ms

After

curl:

$ curl -I -X OPTIONS -H "access-control-request-method: PUT" -H "origin: localhost:8888" "http://localhost:4569/example/abc123"
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: PUT, POST, DELETE, HEAD, OPTIONS
Vary: Origin, Access-Control-Request-Headers, Access-Control-Request-Method
Date: Thu, 29 Nov 2018 21:25:27 GMT
Connection: keep-alive
Content-Length: 0

s3rver:

$ node .
now listening on host 0.0.0.0 and port 4569
info: [S3rver] OPTIONS /example/abc123 200 - - 1.668 ms

[kherock]

Thanks for the PR. Just a minor nit for readability but otherwise it looks good to me.

[kherock]

All right, nice work. @leontastic do you think you could put together another minor release with this and #325 soon?

[leontastic]

@justinmchase Thank you for your contribution. I just cut a new release with this fix: https://github.com/jamhall/s3rver/releases/tag/v2.2.7