Passing Array in REST API Magento 2 (Post Method)

If you take a look at class Mage_Api2_Model_Resource which should be extended by your class, you can find function _render().

By examining dispach() function of this class, you can see, that _render() function is called by retrieve operations. So you can use this function at the end of your action like so $this->_render($data).

This function is not used by create operations, which only set Location header to newly created resource url.

To take an advantage of attribute filtering, use $this->getFilter()->out($data) before $this->_render($data)

Mage_Api2_Model_Resource::dispatch() can answer a lot of questions regarding magento REST!

The interface must have proper dock blocks with the existing types. But your code doesn't have a return type. For example:

<?php 
/**
 * Product type provider
 *
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Catalog\Api;

/**
 * @api
 */
interface ProductTypeListInterface
{
    /**
     * Retrieve available product types
     *
     * @return \Magento\Catalog\Api\Data\ProductTypeInterface[]
     */
    public function getProductTypes();

    /**
     * Retrieve available tests
     *
     * @return \Vendor\Module\Api\Data\TestInterface[]
     */
    public function getTests();
}