WP-API: JavaScript client - fix setup of models used by wp.api.collections objects.

Correct setup for the `model` attribute of `wp.api.collections` objects. Set the collection model as a function that returns a new model of the underlying type, instead of setting it as the model prototype. Fixes an issue where models for fetched collections weren't set up properly and didn't have the expected mixin methods such as `getCategories` available.

Props jesseenterprises.
Fixes #39070.

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git-svn-id: https://core.svn.wordpress.org/trunk@39543 1a063a9b-81f0-0310-95a4-ce76da25c4cd