Transients: Allow a non-expiring transient to be updated with an expiry.

props shahpranaf, sandyr.
fixes #22807.

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Popular tags' edit links should respect the current post type. Adds unit test.

Props mordauk, fahmiadib.
Fixes #25566.


Built from https://develop.svn.wordpress.org/trunk@27720


git-svn-id: http://core.svn.wordpress.org/trunk@27558 1a063a9b-81f0-0310-95a4-ce76da25c4cd