Handle options requests

[koffeinfrei]

Add the possibility to handle OPTIONS requests as well in the handler.

This allows to handle e.g. CORS requests, like the following:

class Handler
  def run(body, headers)
    if headers['REQUEST_METHOD'] == 'OPTIONS'
      return [
        [],
        {
          'Allow' => 'POST, GET, OPTIONS',
          'Access-Control-Allow-Methods' => 'POST, GET, OPTIONS',
          'Access-Control-Allow-Origin' => '*',
          'Access-Control-Allow-Headers' => 'Accept'
        }
      ]
    end

    # ....
end

I also extracted the responding logic to a sinatra helper, as the code wasn't very dry.

[alexellis]

Is there a linked issue for this?

[koffeinfrei]

Is there a linked issue for this?

Yes: #11

I didn't link it initially as I didn't want to create bias for the discussion in the issue (as I created the MR first, which is not according to you contribution guidelines, as pointed out by you).

[alexellis]

Writing the PR first already created a bias.

There is no CORS support in any of the function templates. Having this as a default would go against that, if it's to be added, I would suggest it's made strictly opt-in.

[koffeinfrei]

There is no CORS support in any of the function templates.

Just to make sure we're on the same page: This PR doesn't add CORS support. It just adds a sinatra route for OPTIONS requests, so they are passed to the function as well. What to do with that is still up to the handler function. With this PR GET, POST, PUT, DELETE and OPTIONS requests are processed. Nothing more.

Having said that, do you think passing OPTIONS requests as well could be problematic?