(MODULES-7790) Fix modifying frozen string literal
(MODULES-7790) Fix modifying frozen string literal

In Puppet 6, PUP-7141 changed the 3x runtime to stop duplicating
strings. Providers will now usually receive frozen strings. Update the
concat_file provider to create a new copy of a string if it needs to add
to it rather than attempt to mutate the original.

puppetlabs/puppetlabs-concat