Tolerate `.tar` files that pretend to be `.tar.gz` files

[gaborcsardi]

This happens when a web server claims Content-Encoding: x-gzip but it does not compresses the content.

We could also set

h <- curl::new_handle(accept_encoding = NULL)

which means that libcurl will not send an accept-encoding header, so the server should not claim Content-Encoding: x-gzip.