When gallery settings are overridden, the JS-generated shortcodes need to check the new defaults before deleting attributes that it suspects are the same as the original default values.

`wp.media.collection` has a new method to do this: `setDefaults()`.	Also flips the use of `_.extend` to allow this method to be overriden on instance creation.

See #28693.

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git-svn-id: http://core.svn.wordpress.org/trunk@29066 1a063a9b-81f0-0310-95a4-ce76da25c4cd