Support for dasherized/hyphenized member names

[vovimayhem]

Currently, the payload validators expect member names to be underscored only.

When presented with a payload using either hyphens or spaces (although that's not recommended) which are allowed in the spec, the payload validator will fail.

[vovimayhem]

IF we define the keys using strings with hyphens:

JsonapiSpecHelpers::Payload.register :person do
  key 'first-name'
end

Will make the assertions fail with this exception:

Failure/Error: assert_payload :person, test_person, json_items(0)

     NoMethodError:
       undefined method `first-name' for #<Person:0x0000559cd2ce7628>
       Did you mean?  first_name
                      first_name?
                      first_name=

So, I think I can do something with JsonapiSpecHelpers::Payload#key for this to work for everybody :) I'll send a PR

[ollizeyen]

I've added another try on solving this issue in #24. Please have a look and tell me what you think, @richmolj! Would be great to merge this one since I really need this and don't want to fork.