Is `keep_alive` intentionally not implemented for `header`?

The `keep_alive` function is implemented for `message` but it only requires information from the `header`. Due to this, any function that composes a response header needs to take the whole `message` unnecessarily.

Our work around is to just re-implement the `keep_alive` function and use that, but I'm wondering if there is a reason I'm missing for the exclusion or whether is just hasn't been a priority to have it in `header`.

The issue names the existing message::keep_alive function and the header data it consumes, but no file or test. Start by locating those APIs and checking current response-header composition callers. Done means determining whether the header should expose equivalent behavior and validating the chosen API behavior.

Written by the indexing model from the issue text.