Both `Content-Encoding` and `X-Content-Encoding` are set

As the title suggests, both `Content-Encoding` and `X-Content-Encoding` are set. This is more than likely a bug:
- They are duplicates of one another
- `X-Content-Encoding` is the nonstandard variant of the first

Is it intended for both headers to be set and sent over to the server? Which one does the server check?