[L4] Update closeGroup() to work with Nude framework. Fixes #374

As described in issue #374, the `Former::closeGroup()` method would return a closing `div` tag, even in the Nude framework where the group element is not set.

This pull request updates the code so that `Former::closeGroup()` will call the close method on the current custom group, instead of providing a hardcoded return value.  The method was also updated to make sure that a group is actually open, so that multiple calls to closeGroup() will not print multiple closing tags.

Tests have been added which fail before the code updates and succeed after the code updates.