Generate conditional ETag from inner conditional response headers

Copy `ETag` and `Last-Modified` from the `fetchResp` to the outer `resp`. This enables clients to transparently take advantage of #34 without having to understand the SXG format.

Locate where the inner fetchResp and outer resp are created and their response headers are handled. Start by tracing that response path, then verify that ETag and Last-Modified from fetchResp appear on resp for conditional responses; the issue is done when clients can use those headers without understanding SXG.

Written by the indexing model from the issue text.