Compare copyright prefix in a more encoding safe way

id is a ASCII-8BIT string but the literal "\xa9" could be
UTF-8 string which in that case Ruby won't conceder them equal.
This is probably because "\xa9" is an invalid UTF-8 string etc.

"\xa9".force_encoding(Encoding::ASCII_8BIT) == "\xa9".force_encoding(Encoding::UTF_8) => false
Merge pull request #5 from wader/encoding_safe_copyright_strip

Compare copyright prefix in a more encoding safe way