(#22703) Force encoding of comment value to ASCII-8BIT

The comment field for user resources can often contain non-ASCII
characters. Unfortunately, Puppet retrieves the existing value
(current_value) as ASCII-8BIT, but strings containing non-ASCII
characters are automatically turned into UTF-8, resulting in a failure
to concatenate the strings in change_to_s, with messages such as the
following in Puppet 3.2.4 with Ruby 1.9.3:

  Error: Could not convert change 'comment' to string: incompatible character encodings: ASCII-8BIT and UTF-8
  Error: Could not convert change 'comment' to string: incompatible character encodings: ASCII-8BIT and UTF-8
  Error: /User[rpinson]: Could not evaluate: Puppet::Util::Log requires a message which is due to the fact that current_value is encoded in ASCII-8BIT, but newvalue is in UTF-8 and not "softly" convertible when reaching change_to_s in property.rb.

In order to allow proper management of non-ASCII characters in the
comment field, this PR forces an ASCII-8BIT encoding on the comment
property value in the type.

Note: trying to force the encoding in Puppet code (using e.g.
inline_template) doesn't solve anything, and patching change_to_s to
force the encoding results in the resource not being idempotent anymore,
since comparison fails (but change_to_s doesn't crash anymore).
(#22703) User comment encoding: Do not munge value in place, use encoding constants
(#22703) User comment encoding: Use encoding constant
Merge branch 'raphink-dev/user_comment_ascii'

* raphink-dev/user_comment_ascii:
  (#22703) User comment encoding: Use encoding constant
  (#22703) User comment encoding: Do not munge value in place, use encoding constants
  (#22703) Force encoding of comment value to ASCII-8BIT