A fix in the spirit of PR https://github.com/playframework/playframework/pull/1962, but fixes up a compilation issue that broke the build. The major difference with the API is that instead of passing in a WebDriver class, WebDriver instances are always required. However the default WebDriver instance is of clazz HTMLUNIT. Alternate ones can be passed in using WebDriverFactory(clazz) instead of just clazz as before.