RestBase::post() fix PHPDoc type for $p_payload

In addition to string (json), it can also be array or object.

tests/rest/RestBase.php

@@ -135,7 +135,7 @@ protected function delete( $p_relative_path, $p_query_string ) {
 
 	/**
 	 * @param string $p_relative_path The relative path under `/api/rest/` e.g. `/issues`.
-	 * @param string $p_payload The payload object, it will be json encoded before sending.
+	 * @param mixed $p_payload The payload object, it will be json encoded before sending.
 	 * @return mixed|string The response object.
 	 */
 	protected function post( $p_relative_path, $p_payload ) {