I am guessing this branch was only used with GraphQLWSHandler, and not with
GraphQLTransportWSHandler. That's probably why there were a bunch of bugs. This PR fixes those.

This abstract method is defined in BaseGraphQLTransportWSHandler and is used
instead of `send_json` like in graphql_ws protocol implementation.

This makes multi-part post requests work in cases where one or more files are
being sent with the graphql query.

This is done to enable passing in utf-8 filenames in headers.

to add support for persisted queries, we want to bypass repeated validation of
queries that have already been parsed and validated.

This allows that by letting the user pass in the `DocumentNode` directly which
bypasses much of the parsing and validation, making the query processing faster.