Signature: Enable digest check for REST Requests

[obenland]

The digest check was essentially skipped, even if a digest header was available, since $body was never set. I suppose there's a good chance that requests might fail now that didn't use to fail? Not sure.

Proposed changes:

• Enables Digest check for WP_REST_Requests.
• Uses signed headers directly instead of in a separate variable.
• Breaks down digest in separate variables.
• Adds tests.

Other information:

• [x] Have you written new tests for your changes, if applicable?

Testing instructions:

• Apply this PR to a test site that can receive requests from the internet.
• Send a request that includes a digest, like a like from Mastodon maybe?
• Make sure that still works.

Changelog entry

• [x] Automatically create a changelog entry from the details below.

Changelog Entry Details

Significance

• [x] Patch
• [ ] Minor
• [ ] Major

Type

• [ ] Added - for new features
• [ ] Changed - for changes in existing functionality
• [ ] Deprecated - for soon-to-be removed features
• [ ] Removed - for now removed features
• [x] Fixed - for any bug fixes
• [ ] Security - in case of vulnerabilities

Message

When verifying signatures on incoming requests, the digest header now gets checked as expected.