UStar tar archives have a `magic` header field at byte offset 257 in
each entry whose value begins with the string `ustar`. Identify them
with the MIME type `application/x-tar`.

Also add test cases for a number of UStar-compatible formats, created by
GNU tar 1.29 (with `--format=<format-name>`):

* `tar.gnu.tar`
* `tar.oldgnu.tar`
* `tar.posix.tar`
* `tar.ustar.tar`

as well as `tar.star.tar` (created by star 1.6) and, for completeness,
`tar.v7-gnu.tar` (a v7 tar archive created by GNU tar 1.29).

Fixes gabriel-vasile#307.