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This is a different fix for bug#34909, which should also fix bug#35739.

Our downloading code used to automatically decode the result according
to the usual heuristics for files.  This caused problems when we later
needed to save the data in a file that needed to be byte-for-byte
equal to the original in order to pass the signature verification,
especially because we didn't keep track of which coding-system was
used to decode the data.

(package--unless-error): New macro extracted from
package--with-response-buffer-1, so that we can specify edebug and
indent specs.
(package--with-response-buffer-1): Use it.  More importantly, change
code so it runs `body` in a unibyte buffer with undecoded data.
(package--download-one-archive): Don't encode with utf-8 since the data
is not decoded yet.
(describe-package-1): Explicitly decode the readem.txt files here.

* lisp/url/url-handlers.el (url-insert-file-contents): Use it.
(url-insert): Don't decode if buffer is unibyte.

* lisp/url/url-http.el (url-http--insert-file-helper): New function,
extracted from url-insert-file-contents.