GraphQlView: Do not 'instantiate_middleware' if middleware is already a MiddlewareManager

[msimon]

In graphq-core 2, the MiddlewareManager can have a wrap_in_promise boolean passed to activate/deactivate wrapping the middleware's result into a promise. This has a big performance impact on large requests.
https://github.com/graphql-python/graphql-core-legacy/blob/52e030bbc6001fd378515600f81ccea0e360131b/graphql/execution/middleware.py#L25

The executor is instantiating the middleware as a MiddlewareManager if it's anything else than already a MiddlewareManager:
https://github.com/graphql-python/graphql-core-legacy/blob/52e030bbc6001fd378515600f81ccea0e360131b/graphql/execution/executor.py#L102

So in graphene_django/views.py by always converting middleware into a list, it breaks the ability to passe the argument to the MiddlewareManager and always default to wrap_in_promise=True.

This PR simply checks if the middleware is already a MiddlewareManager and pass it without pre-processing, allowing the end-user to define it themselves.

[jkimbo]

Thanks @msimon