See gh-996

This commit aligns DEBUG logging of exceptions resolved via annotated
exception handler methods with ExceptionResolversExceptionHandler and
the reference documentation.

Closes gh-996

Prior to this commit, gh-948 added a fallback support for the
"application/graphql" content-type sent by clients. This media type is
not widely used and advised against by the spec group.

This fallback checked for an exact match of the content type, not taking
into account potential media type parameters such as charset.

This commit ensure that a `MediaType#include` comparison is used to
trigger the fallback.

Fixes gh-1038

Prior to this commit, the `DefaultExecutionRequestObservationConvention`
would only produce "INTERNAL_ERROR" outcomes if the response is null or
if an unresolved exception remains.

The `ExceptionResolversExceptionHandler` will catch all unresolved
exceptions and add them to the errors map with the
`ErorType.INTERNAL_ERROR` error type. This means that the
"INTERNAL_ERROR" outcome is never used.

This commit ensures that this outcome is also used if at least one
resolved error is of `ErorType.INTERNAL_ERROR`.

Fixes gh-1065

Prior to this commit, `@BatchMapping` controller methods would be
automatically registered as date fetchers delegating to data loader
calls. Those calls would not include the current local context or main
context. As a result, injecting the `BatchLoaderEnvironment` in the
controller method signature would not contain the `getKeyContext()`.

This commit ensures that dataloader calls not only use the current
source, but also the current local context/main context so that it will
be present in the key contexts map.

Fixes gh-1071

Closes gh-1072

Closes gh-1073

See gh-1066