Use string representations in cartesian().

This makes the cartesian() function handle some of the data types
added in recent versions of Puppet without crashing.

Example, when using Puppet 4:
  cartesian('foo', [1, 2]) returns ['foo1', 'foo2']

Signed-off-by: Thomas Bellman <bellman@nsc.liu.se>

This was originally commit a11ce806b001 in nsc-puppet-utils.