Align GraphQL Response Content Types and Status Codes with Specification

[ujibang]

Describe the bug

The GraphQL API is not consistent with the GraphQL Over HTTP specs

Specifically:

The server SHOULD NOT use a 4xx or 5xx status code for a response to a well-formed GraphQL-over-HTTP request.

To Reproduce

Steps to reproduce the behavior:

1. Execute the test case "Should contains errors key" in field-to-aggregation.feature
2. The status code is 400

Expected behavior

The response codes should adhere to the specifications.

Detailed documentations

the response content type from the GraphQL API is now always application/graphql-response+json

the API response codes are:

• 404 graphql app not existing
• 405 wrong method (not POST or OPTIONS)
• 500 connection error with mongodb <== this is the only case when 500 is returned
• 200 when valid GraphQL response even if contains some errors (partial data)
• 400 when the request is invalid (e.g. wrong json, wrong GraphQL query, not existent fields in selection, etc)
• 400 when the request is valid but the response only contains errors the prevented any data to be fetched (i.e. data: null)

For the last case the specification says:

If the GraphQL response does not contain the {data} entry then the server MUST reply with a 4xx or 5xx status code as appropriate

[jodeleit-es]

I assume the first link has a wrong target. Did you want to link to this URL? --> https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md#applicationjson