[38592] changed the behaviour of `wpautop()` , so it's nice to change `autop()` to match.
Interestingly, this change isn't necessary for the functionality to work - #4857 didn't affect content that had been run through `autop()` at some point, as `autop()` would add a single line break before block elements, then `wpautop()` would later add a second line break, making it work correctly.
Props nacin for finding out about [38592] on Twitter, and DMing me to remind me to review `autop()`.
See #4857.
Built from https://develop.svn.wordpress.org/trunk@38594
git-svn-id: http://core.svn.wordpress.org/trunk@38537 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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