REST API: On comment create, fallback to the `user_agent` header value.

If a user-agent is not explicitly provided in the `author_user_agent` parameter, fallback to the `user_agent` value in the request header.

Props dd32, jnylen0, rachelbaker.
Fixes #38817.
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git-svn-id: https://core.svn.wordpress.org/trunk@39227 1a063a9b-81f0-0310-95a4-ce76da25c4cd