set_content() did not drop a content provider installed earlier, and
apply_ranges() then took the body's size as the Content-Length while the
provider was still the one written. The provider was asked for more bytes
than it had: a file under a mount point went out under the error status a
file request handler set to hide it, its mapping was read past the end,
and a user provider read past its own data.
Make every content setter clear the other sources so the last one wins,
let a body assigned directly after a provider or a file win as well, and
never read past the mapping in the file content provider.