`fetch` decompresses a response body before anyone sees it, and the headers it
returns still say `content-encoding: gzip`. Copying those onto the decoded bytes
tells the client to gunzip plain JSON: `Decompression error: ZlibError`, which
surfaced as a 502 from the catch that wraps the forward.
It hides well, and that is the part worth writing down. A 401 body is a few bytes
and is not compressed, so every unauthenticated probe passes. The first request
carrying a real credential is the first one big enough to be gzipped, and the
first one to fail - which reads as "the credential broke it".
Measured on alpha in exactly that order: the extension door answered 401 without
a token and 502 with one.
`content-length` goes too. It describes the compressed length and the body being
handed on is not that length.
Response headers are now a different function from request headers, because they
are a different question and treating them as one is what caused this.
pastilhas
ed40b6798c