Custom context support

[TheRusskiy]

Hello!
Thank you for turning this into a gem.
I was wondering if there's a way to access custom context.

For example when I do

class MySchema < GraphQL::Schema
  context_class MyContext # <====
  ...
  use GraphQL::Batch
  enable_preloading
end

and later do

preload_scope: -> (args, context) {
    puts context.class.name
    ...
  }

I am getting
"GraphQL::Query::Context::FieldResolutionContext"
instead of
"MyContext"

FYI my ruby-graphql version is 1.9.3.

Thank you again!