[awc] Fix response body when no content-length header

[armandmgt]

PR Type

Bug Fix for awc

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the latest stable rustfmt.
• (Team) Label with affected crates and semver status.

Overview

As noted in #2287 response body should be read until connection is closed when neither Transfer-Encoding nor Content-Length are specified in HTTP/1.1 (item 7 in https://www.rfc-editor.org/rfc/rfc7230#section-3.3.3).
In order not to break any current tests I added a check for the presence of the Connection: close header (which is not required by the spec).
For reference this is how go's standard library is doing it: https://github.com/golang/go/blob/aeb0644bd33e67f24e2411a651ac9ff72ddc96b4/src/net/http/transfer.go#L548

[armandmgt]

Not sure this is really relevant / up-to-date
Feel free to re-open if needed !