Support client requests with content-type "application/graphql"

[rstoyanchev]

It's not a media type from GraphQL over HTTP spec, but it's used widely enough by clients and some tooling, e.g. IntelliJ uses it in the HTTP client. However, message converters and codecs are only configured for "application/json". We need to be more lenient, and allow such requests to be handled.

[github-actions]

Fixed via 7dd9cf5

[rstoyanchev]

GraphQlHttpHandler implementations now check for "application/graphql" defensively, if we fail to decode due to an unsupported media type, and then try again with "application/json". This fallback approach is justified given the general direction of the spec which expects "application/json" and decreasing use of "application/graphql" judging from graphql/graphql-over-http#249 (comment) for example.

That makes the handlers capable, but the router mappings also need an update. For 1.3 there is a new GraphQlRequestPredicates based on #906 that needs to also be updated to allow "application/graphql", so I'm re-opening this issue to do that. For 1.2 the mapping is in the Boot starter, so we can't change that here.