Set $variation object when add variation via ajax

When the variation-admin-html.php template is loaded on the Edit
Product screen, $variation is a WP_Post Object. When the template
is loaded by woocommerce_add_variation() handling an Ajax call,
$variation is an array. This commit makes $variation consistent so
functions hooking to 'woocommerce_product_after_variable_attributes'
and 'woocommerce_variation_options' receive the same variable type
regardless of what initiated the template loading.

woocommerce-ajax.php

@@ -655,6 +655,7 @@ function woocommerce_add_variation() {
 
 		$_tax_class = '';
 		$image_id = 0;
+		$variation = get_post( $variation_id ); // Get the variation object
 
 		include( 'admin/post-types/writepanels/variation-admin-html.php' );
 	}