We goofed, and parameters accessed through the REST API's methods
were slashed (inconsistently, even). This unslashes the data, so
you get the un-messed-with data that was sent.
Props joehoyle.
Fixes#36419.
git-svn-id: https://develop.svn.wordpress.org/trunk@37163 602fd350-edb4-49c9-b593-d223f7449a82