`is_feed()` check is already done in embed_rss() function, no need to double it
Mini-improvements to embed_rss test
Add clarification comment to function
Prevent `DateTime` borking if int is passed to it.

`null` will make it return the current date & time
Minified the metabox.js file

Fixes issue #1312, #1313
Undo unit test update
Merge pull request #1316 from jrfnl/Mini-fixes

Mini fixes, props @jrfnl