Add headers to return

[jnutter]

Return headers to promise resolve.

Use case: When fetching pagination data, our API returns a next page header.

[jnutter]

Do you think its better to return the whole response instead? I would like the response.ok as well as err.

[ryanashcraft]

@jnutter You'll need to update this PR because of the changes from #23.

Let's call the field headers when supplied to the promise callback. Additionally, can you also go ahead and start including it in the REQUEST_SUCCESS / REQUEST_FAILURE / MUTATE_SUCCESS / MUTATE_FAILURE actions with the field name responseHeaders? See #39 for a reference here.

I want to avoid passing variables that are too specific to superagent. You should be ok (pun intended 😝) with computing ok from status in your app. See the "Response status" section in superagent's docs for how it computes response.ok.

[jnutter]

@ryanashcraft please let me know if there is anything else 😄

[ryanashcraft]

thanks for contributing again @jnutter!