Ruby 1.9.3-p0 support in spec tests.

Ruby 1.9.3-p0 will invoke to_ary on every element of an Array when
Array#flatten is invoked, which Ruby 1.8.7, and Ruby 1.9.3-p125 will not.

This leads to a failure in this test because our stub objects explode when you
invoke that method, rather than the normal Ruby behaviour. Just stubbing that
method to return nil will do the right thing.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>