http: ensure HEAD and GET requests have same headers

[tonistiigi]

fixes #3739

Currently, GET requests get Accept-Encoding: gzip added in stdlib, but for HEAD requests, that does not happen https://github.com/golang/go/blob/go1.20.2/src/net/http/transport.go#L2575 . Github server will return different Etag headers based on different request headers(likely handled by separate services), and does not match If-None-Match against Etag from different request.

[jedevc]

Would it be worth adding a test for this to compare headers from GET and HEAD (I assume the other headers are similar)?

Ideally, we don't want to have a regression if the stdlib changes behavior and we don't notice the change.

[tonistiigi]

Added test.