Prevent double purge on post

[RavanH]

Sometime update post status hook may run twice. See https://wordpress.org/support/topic/purge-on-publish-sometimes-runs-twice/

[chandrapatel]

@RavanH Apologies for the late reply. When the user updates the already published post then the old and new status will be publish and in that case this PR change won't purge the post after post update. Let me know if I'm missing something here.