in_http: Implement support for CORS preflight requests

[fujimotos]

This patch should fix the issue #1701.

To receive a 'application/json' request from another origin, we need
to be able to handle CORS preflight requests. Otherwise, browsers will
refuse to interact with us.

https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#Preflighted_requests

This patch adds support for the feature.

Signed-off-by: Fujimoto Seiji [email protected]

[fujimotos]

I confirmed this patch works with Fluentd v1.2.6 and Firefox 60.2.1.

Test Code (JavaScript)

// Post a JSON record using XMLHttpRequest
var req = new XMLHttpRequest();
req.open('POST', 'http://fluentd.local/debug.log');
req.setRequestHeader('Content-Type', 'application/json')
req.send(JSON.stringify({'foo': 'bar'}))

Result

[repeatedly]

Thanks!