- rework `rocket_server` example as `simple`
- provide example in `GraphQLRequest` API docs
- mention GET query format in `GraphQLRequest` API docs and `simple` example
Additionally:
- fix `operationName` query parameter handling
- make `graphiql_source()` and `playground_source()` polymorphic over `subscriptions_endpoint_url` argument
- provide examples in `graphiql_source()` and `playground_source()` API docs
- move integration HTTP tests to a separate file
- test both sync and async `juniper_rocket` in integration HTTP tests
- polish `FromForm` unit tests