pure texinfo files: do we need to do anything special about encoding?

[cbm755]

See #254. We read these directly using fileread:

  if any(strcmpi(ext, {'.texinfo' '.texi' '.txi' '.tex'}))

Anything special need be done to ensure we get a utf-8 internal string directly from that?

Low priority: I don't think folks are using doctest extensively with .texinfo files. I also don't know how prevalent non-ascii texinfo files are.