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ARROW-4390: [R] Serialize "labeled" metadata in Feather files, IPC messages

The work for this was done in ARROW-8899; this just adds a test confirming the expected behavior on types created by the `haven` package.

Closes #7600 from nealrichardson/haven

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>