New custom_method DSL for defining custom API request methods as static methods

[ob-stripe]

r? @brandur-stripe @remi-stripe

This PR adds a new custom_method DSL that can be used in API resources to define a class method for custom API request methods. custom_method takes 3 parameters:

• the name of the method
• the HTTP verb
• the path to append to the URL. This is optional, if not provided, the name of the method is used as the path

For instance, on Stripe::Charge:

custom_method :capture, :post

is equivalent to:

def self.capture(id, params = {}, opts = {})
  url = "#{resource_url}/#{CGI.escape(id)}/capture"
  resp, opts = request(http_method, url, params, opts)
  Util.convert_to_stripe_object(resp.data, opts)
end

This should make it easier to transition to static methods and deprecate instance methods (cf. #752).

I've added custom_method everywhere where it should apply (though I may have missed some), and added tests for some (not all) of the new methods.

I've also added a Stripe::Discount class for discount objects.

[brandur-stripe]

Nice OB!

Left a couple comments, but looks great.

ptal @ob-stripe

[ob-stripe]

Made the requested changes, ptal @brandur-stripe.

[brandur-stripe]

Awesome OB! Looks great to me.

[ob-stripe]

@remi-stripe Want to take over the branch to check that I haven't missed any custom methods and add the tests that are missing?

[remi-stripe]

@ob-stripe Also pushed changes to the tests.

[remi-stripe]

@ob-stripe I just realized we forgot to do all the namespaced resources like terminal and such too

[remi-stripe]

re-assigning to ob to finish reviewing and release

[remi-stripe]

@ob-stripe turns out I only messed up 2 of them!!!! Re-assigning to you again :p

[ob-stripe]

LGTM!

[ob-stripe]

Released as 4.12.0.