[webtransport] opening a bidirectional stream triggers exception

From the test page https://googlechrome.github.io/samples/webtransport/client.html, when opening a bidirectional webtransport stream, an exception is triggered that prevents from receiving data from the server.

To reproduce, from https://googlechrome.github.io/samples/webtransport/client.html
- connect to your webtransport server
- enter some content in the textarea input, select `Open a bidirectional stream` then click `Send data`

**result:**
- a bidirectional webtransport stream is established, data can be sent but not received
- in the JS console, the exception below is triggered:
`Uncaught (in promise) TypeError: stream.pipeThrough is not a function`

**what is expected:**
- a bidirectional webtransport stream is established, data can be sent and received and no exception is shown in the JS console (I'm testing with a simple webtransport server that echoes back any received data)