WIP: allow the user to disable HTTP 500 if the response contains only errors

[julkwiec]

By default, the express-graphql library changes the HTTP response code to 500 if the response.data property is falsy.
As a back-end developer, I want to be able to decide whether to respond with HTTP 200 or HTTP 500, even if the response contains no data, just errors.

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[coveralls]

Coverage increased (+0.002%) to 99.375% when pulling f88872c on julkwiec:custom-http-status-code into 750cfd5 on graphql:master.

[lanzanka]

@IvanGoncharov Hi! Could you review this pull request? Do you know what the estimated date of the next release is?

[nvieirarafael-sensedia]

This option would be great!
I recently update my project to use latest version of the lib and these 500 status code on queries without data are giving me integration problems with clients that already use api.

[julkwiec]

Fixed merge conflicts.

Also, #471 seems like a better solution ;)