Thread request context through to GraphQL executor

[bendemaree]

This addresses the problem raised in #7; when graphql() is invoked, the current context value is not being passed to the executor (via context_value).

Here's a straightforward way to reproduce; if you run with and without the patch here, you'll see two different things printed:

import graphene


class Query(graphene.ObjectType):
    noop = graphene.String()


class Subscription(graphene.ObjectType):
    ping = graphene.String(required=True)

    async def resolve_ping(root, info):
        print(info.context)
        yield "pong"

    async def resolve_player_statistics(root, info):
        pass


schema = graphene.Schema(query=Query, subscription=Subscription)

and (using WsLibSubscriptionServer):

async def handle_subscription(ws, path):
    await subscription_server.handle(ws, request_context={"foo": "bar"})

This will print None and {'foo': 'bar'} before and after this patch, respectively.

[SmileyChris]

Fixed