Accept-Encoding in Variants

When fetching a Signed Exchange, Chromium sets Accept-Encoding request header to gzip, deflate, br. However, actually Chromium accepts differnt set of content encodings for Signed Exchange's outer and inner responses. gzip, deflate, and br are supported for outer responses, and only mi-sha256-03 is supported for inner responses.

This is problematic when performing the Request Matching. For example if exchange has:

 Variants: Accept-Encoding;mi-sha256-03
 Variant-Key: mi-sha256-03

It will not match the browserRequest which has Accept-Encoding: gzip, deflate, br.

What should we do?

Ideas:

• When making a redirect to the inner response, overwrite the request's Accept-Encoding header to mi-sha256-03.
• Or, specify that Cache Behavior for Request Matching does not support Accept-Encoding content negotiation, because it makes little sense for signed exchange's inner responses (?)

No files, tests, or implementation entry points are mentioned. Start by reviewing the Request Matching and Cache Behavior sections linked in the issue, then determine which Accept-Encoding behavior is intended for Signed Exchange outer and inner responses. Done means the project has an agreed specification or implementation direction for the mismatch.