(maint) Dup frozen File content strings

Commit 2a9e996 froze all string tokens in the lexer as a performance
tweak. This caused a regression in file content behavior where

file { '/tmp/foo': content => stuff } would fail with a 'Error: Could not
set content on file: can't modify frozen String' when using the future
parser on Ruby 1.9.3 and up.  The Ruby version requirement was because
we force_encoding to ASCII_8BIT on platforms (1.9.3 and up) which allow
this, but you can't do that to a frozen string.  This was failing in the
lib/puppet/type/file/content.rb should= method, where the string was
cloned, but a clone() clones the frozen state.

Changed to a dup() which provides an unfrozen copy, and added a spec.