public function send(array $campaign)
    {
        $this->assertValidCampaign($campaign);

        $request = new Request('campaigns');
        $request->addSubresource(['name' => 'send']);
        $request->setParams($campaign);

        $response = $this->client->post($request);

        $this->assertSuccessResponse($response);

        return $response->getData();
    }