The following cases are excluded in the patch:
- The Firefox PDF Viewer, since it has been fixed on the platform side already; please see https://bugzilla.mozilla.org/show_bug.cgi?id=1683940
- The `PDFNodeStream`-implementation, used in Node.js environments, since after recent changes that code only supports `file://`-URLs.
Also updates the `PDFNetworkStreamFullRequestReader.read`-method to await the headers before returning any data, similar to the implementation in `src/display/fetch_stream.js`.
*Note:* The relevant unit-tests are updated to await the `headersReady` Promise before dispatching range requests, since that's consistent with the actual usage in the `src/`-folder.
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