Actix integration: cbor response support + error handling improvements

[Follpvosten]

This PR does two things:

• Remove .unwrap() from the Responder impl for GraphQLResponse, instead opting to act the same way Actix's Json responder does it: https://docs.rs/actix-web/4.0.0-beta.20/src/actix_web/types/json.rs.html#119-137
• Add cbor support to the same impl, reading the Accept header and acting based on the mime type in there.

I'm not 100% sure if this PR is in a mergable state, I could feature-flag the whole cbor thing or split this up into two PRs if that's preferred.

I've also noticed that the cbor request implementation doesn't actually support the application/cbor mime type atm, the stated reason being that mime doesn't support it yet; however mime is unmaintained and will never add it, and also it's possible to match on (mime::APPLICATION, "cbor") without the crate needing to add support. That's definitely a separate PR however (and I don't need cbor requests, so I'm not sure if I'm the right person to implement that).

[sunli829]

Thank you! It would be nice to have serde_cbor as an optional feature. 🙂

[Follpvosten]

Alright, a couple notes:

• While I'm much more satisfied with this version, there might be some things which are still missing or could be improved
• I've basically copied the add test and adjusted it to request cbor. Because serde_cbor doesn't have a macro like serde_json's json!, I've derived Deserialize on a struct for that.

[sunli829]

Nice, I'll merge it first and then refactor if necessary. I would like to move the cbor response to async-graphql so that all integrations can be easily supported.