Fix compatibility with Selenium 2.x

Author: mvorisek

lib/WebDriver/Session.php

@@ -476,7 +476,13 @@ private function serializeArguments(array $arguments)
         foreach ($arguments as $key => $value) {
             // Potential compat-buster, i.e., W3C-specific
             if ($value instanceof Element) {
-                $arguments[$key] = [Container::WEBDRIVER_ELEMENT_ID => $value->getID()];
+                // preferably we want to detect W3C support and never set nor parse
+                // LEGACY_ELEMENT_ID, until detection is implemented, serialize to both
+                // variants, tested with Selenium v2.53.1 and v3.141.59
+                $arguments[$key] = [
+                    Container::WEBDRIVER_ELEMENT_ID => $value->getID(),
+                    Container::LEGACY_ELEMENT_ID => $value->getID(),
+                ];
                 continue;
             }