Historically, the REST API would generate the entire response object, including running expensive filters, then it would apply the `_fields` parameter, discarding the fields that weren't specificed.
This change causes `_fields` to be applied earlier, so that only requested fields are processed.
Props danielbachhuber.
See #43874.
git-svn-id: https://develop.svn.wordpress.org/trunk@43087 602fd350-edb4-49c9-b593-d223f7449a82
4ac3f4c13a