Fix #6280 - puppetdoc crashing on string interpolation

The following manifest was crashing puppetdoc:

class test {
   include "test::$operatingsystem"
}

Because the quoted string is "rendered" as a concat AST, which in turn
ended being an array when entering RDoc.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>