OPTIONS is a noop request that is used by the browsers
to check if server will accept cross-site XMLHttpRequest
(indicated by the presence of CORS headers)

Before this fix user could enable CORS headers in the Gateway config,
but XHR failed due to the lack of support for OPTIONS request type
(as described in https://git.io/vzgGe)

License: MIT
Signed-off-by: Marcin Rataj <lidel@lidel.org>

- Implements
  #2232 (comment)
- Separate test suite:
    - we don't want to pollute other gateway tests with CORS headers
    - (as of now) changing headers requires daemon restart anyway

License: MIT
Signed-off-by: Marcin Rataj <lidel@lidel.org>