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`url-insert-file-contents` saves in buffer-file-coding-system
the coding-system used to decode the contents.  Preserve this
as the contents is moved from buffer to string to buffer, and use
it when saving the contents to file, so as to try and better preserve
the original byte sequence.

(package--buffer-string, package--cs): New functions.
(package--check-signature): Encode `string` if a coding-system
was specified in buffer-file-coding-system.
(package--download-one-archive, package-install-from-archive):
Obey and preserve the buffer-file-coding-system if specified.

Do not merge.