* Don't compress a response whose handler already set Content-Encoding
* Don't compress a pre-encoded response served from a file
The guard that stands down when a response already names a content coding
covered the responses that settle their coding in `apply_ranges()`, but a
file-backed one settles it in `static_file_encoding()`, which asked the
content-type overload and so never saw the field. With static file
compression enabled, a mount point naming the coding for a tree of
build-time compressed assets, and a handler setting the field on a
`set_file_content()` response, both had their stored bytes compressed a
second time and a second `Content-Encoding` field line appended.
A file-backed response has not been given a content type by the time its
coding is decided, which is the only reason it could not go through
`encoding_type()`. It takes the type as an argument now, so both paths share
the one guard instead of carrying a copy each.
`Response::content_encoding_` becomes `content_coding_`, after what it
holds. It names the coding chosen for the body, which is what its own
comment already called it, while the old name read as the value of the
`Content-Encoding` field whose presence is exactly what forces the coding to
`None`.
README gains the behaviour, including the part that stays with the handler:
`Vary` is added only to a coding the server chose, so a handler that picks a
representation from `Accept-Encoding` has to add the field itself.
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Co-authored-by: yhirose <yuji.hirose.bug@gmail.com>
