Spec compliance

[hoangvvo]

This issue tracks the process of making Benzene libraries spec-compliance.

@benzene/http needs to be compliant with https://github.com/graphql/graphql-over-http and @benzene/ws needs to be compliant with https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md.

Overall, it is fairly good to go but we might need unit tests to confirm it.

http

• Accepts application/graphql+json content type (Add support for application/graphql+json content type #71)
• Respond with content type application/graphql+json and consider Accept header.

ws

TBD