(MODULES-707) chomp() fails because generate() no longer returns a string

We need to use

  unless value.is_a?(String) || value.is_a?(Array)

rather than

  klass = value.class
  unless [String, Array].include?(klass)

because the klass version enforces type checking which is too strict, and does
not allow us to accept objects wich have extended String (or Array).

For example, generate() function now returns Puppet::Util::Execution::ProcessOutput
which is just a very simple extension of String.  While this in it's self was
not intentional (PUP-2306) it is not unreasonable to cope with objects which
extend Strings
Re-use existing str2bool code rather than doing a copy and paste
Merge pull request #262 from tremble/MODULES-707

Modules 707