Fixes exception thrown when using UiHelperTrait functions (i.e. drupalGet, etc.).

Functions from UiHelperTrait (like drupalGet) are used in the Selenium test examples, but they don't work because with HTML output enabled they try to grab response headers which is not supported by the Selenium driver. The examples should probably be updated also, but this is a start at addressing the problem. This is probably better fixed in Drupal core by adding a Selenium2Driver check, similar to how those functions check for GoutteDriver.