Source

File: wp-includes/Requests/Session.php

protected function merge_request($request, $merge_options = true) {
		if ($this->url !== null) {
			$request['url'] = Requests_IRI::absolutize($this->url, $request['url']);
			$request['url'] = $request['url']->uri;
		}

		if (empty($request['headers'])) {
			$request['headers'] = array();
		}
		$request['headers'] = array_merge($this->headers, $request['headers']);

		if (empty($request['data'])) {
			if (is_array($this->data)) {
				$request['data'] = $this->data;
			}
		}
		elseif (is_array($request['data']) && is_array($this->data)) {
			$request['data'] = array_merge($this->data, $request['data']);
		}

		if ($merge_options !== false) {
			$request['options'] = array_merge($this->options, $request['options']);

			// Disallow forcing the type, as that's a per request setting
			unset($request['options']['type']);
		}

		return $request;
	}