fix(ClientRequest): do not unshift write buffer when flushing request headers

[kettanaito]

• Closes fix: MockHttpSocket: if passthrough: dont flush write buffer on stream read #659
• Fixes Bypassed POST request yields invalid on-the-wire HTTP request (missing starting-line) with form-data file attachment msw#2309

Changes

Removes this.writeBuffer.unshift() when flushing request write buffer on the first read() of the request body stream. This will preserve the first chunk to be sent to the original server in case of passthrough.

The fix itself is to call all the write callbacks in order but preserve this.writeBuffer so it's forwarded in its entirety over the original socket.

[kettanaito]

Released: v0.36.10 🎉

This has been released in v0.36.10!

• 📄 Release notes
• 📦 npm package

Make sure to always update to the latest version (npm i @mswjs/interceptors@latest) to get the newest features and bug fixes.

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