use "external" encoding (set by locale, usually UTF-8) for YCP strings

instead of ASCII-8BIT default which can lead to error
"incompatible character encodings: ASCII-8BIT and UTF-8" when
merging Ruby and YCP strings
Merge pull request #24 from lslezak/external_string_encoding

Use "external" encoding (set by locale, usually UTF-8) for YCP strings