Ruru headers are not passed along with the `introspectionQuery`
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### Summary

Ruru does not pass the custom headers with the `introspectionQuery`.

### Steps to reproduce

1. Open ruru
2. Add a header to the headers field
3. Refresh the page

### Expected results

The introspection `POST` should contain the custom headers. This was the behaviour of Graphiql before Ruru.

### Actual results

Custom headers are not present in the `introspectionQuery`.

### Additional context

Ruru version 2.0.0-beta.7.
Postgraphile version 5.0.0-beta.15

Graphiql also used to send the headers as the `payload` portion of the `connection_init` message sent by the websocket.

