Improve frame context management when nested withFrame() calls are used

Now that support for switching to parent frame has been added to web driver via `WebDriver.TargetLocator.parentFrame()` use that instead of `WebDriver.TargetLocator.defaultContent()` to switch out of the frame at the end of `withFrame()` method. This will mean that after a nested call to `withFrame()` the context will be reverted to that of the parent frame and not the top level of the document.
