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.

Built from https://develop.svn.wordpress.org/trunk@39603


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

wp-includes/js/wp-api.js

@@ -1234,7 +1234,9 @@
 						},
 
 						// Specify the model that this collection contains.
-						model: loadingObjects.models[ modelClassName ],
+						model: function( attrs, options ) {
+							return new loadingObjects.models[ modelClassName ]( attrs, options );
+						},
 
 						// Include a reference to the original class name.
 						name: collectionClassName,
@@ -1257,7 +1259,9 @@
 						url: routeModel.get( 'apiRoot' ) + routeModel.get( 'versionString' ) + routeName,
 
 						// Specify the model that this collection contains.
-						model: loadingObjects.models[ modelClassName ],
+						model: function( attrs, options ) {
+							return new loadingObjects.models[ modelClassName ]( attrs, options );
+						},
 
 						// Include a reference to the original class name.
 						name: collectionClassName,

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-39601';
+$wp_version = '4.8-alpha-39603';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.