ubus: support GET method with CORS requests

ubus: support GET method with CORS requests

Complex GET requests (e.g. those with custom headers) require browsers
to send preflight OPTIONS request with:
Access-Control-Request-Method: GET

It's important to reply to such requests with the header
Access-Control-Allow-Origin (and optionally others) to allow CORS
requests.

Adding GET to the Access-Control-Allow-Methods is cosmetical as
according to the Fetch standard:

> If request’s method is not in methods, request’s method is not a
> CORS-safelisted method, and request’s credentials mode is "include" or
> methods does not contain `*`, then return a network error.

It basically means that Access-Control-Allow-Methods value is ignored
for GET, HEAD and POST methods.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>