JWS url calculation can't handle absoluteURI form Request-URIs

https://tools.ietf.org/html/rfc2616#section-5.1.2

To allow for transition to absoluteURIs in all requests in future
versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI
form in requests, even though HTTP/1.1 clients will only generate
them in requests to proxies.

Curiously, this seems to have come up twice in recent times on the forums.

POST https://192.168.1.112:14000/sign-me-up HTTP/1.1
Host: 192.168.1.112:14009
User-Agent: eggsampler-acme/v3 Go-http-client/1.1
Content-Length: 548
Content-Type: application/jose+json
Accept-Encoding: gzip

{"protected":"eyJhbGciOiJFUzI1NiIsImp3ayI6eyJjcnYiOiJQLTI1NiIsImt0eSI6IkVDIiwieCI6IjBXamdrcVlnTk1uVDlUeThURml5N2VibWFJdU05V1AwQUdiZnZBTmFTZmMiLCJ5IjoiSzJIX3NTdGozOFVMUlc2Vml0VUJ0RkIzY0NGTGR1YTN6Z3otbUkyUTF1cyJ9LCJub25jZSI6IkNRVE9VRDg1QlBsaXVKX0FnVTB6enciLCJ1cmwiOiJodHRwOi8vMTkyLjE2OC4xLjExMjoxNDAwOS9zaWduLW1lLXVwIn0","payload":"eyJvbmx5UmV0dXJuRXhpc3RpbmciOmZhbHNlLCJ0ZXJtc09mU2VydmljZUFncmVlZCI6dHJ1ZSwiZXh0ZXJuYWxBY2NvdW50QmluZGluZyI6bnVsbH0","signature":"_oy0VNfWsxVk_e11WR-dm0T6raI6n-juBiCrzd-ACprfbJDD2OqmYhkxP43T7NsIY6ssdeoMS0dWtNgUXXqr0A"}


HTTP/1.1 400 Bad Request
Cache-Control: public, max-age=0, no-cache
Content-Type: application/problem+json; charset=utf-8
Link: <http://192.168.1.112:14000/dir>;rel="index"
Replay-Nonce: FT1FqcM6jzBo_o6HWOGtcQ
Date: Wed, 17 Mar 2021 22:14:55 GMT
Content-Length: 252

{
  "type": "urn:ietf:params:acme:error:malformed",
  "detail": "JWS header parameter 'url' incorrect. Expected \"https://192.168.1.112:14000/https://192.168.1.112:14000/sign-me-up\", got \"http://192.168.1.112:14009/sign-me-up\"",
  "status": 400
}

(Excuse the scheme mismatch, I hacked Pebble a little to make packet captures easier, I promise the same thing happens on master).

I checked and Let's Encrypt is unaffected, probably because the nginx frontend or whatever is dealing with it. Bare Boulder might have the same behavior as Pebble, but I didn't check.

The issue names no files or tests. Start by reproducing the request with an absoluteURI Request-URI, then trace Pebble's JWS URL calculation and validation entry point. Done means this request form is accepted and the validated URL matches the intended endpoint rather than being prefixed with the server URL.