Ruby 2.0.0 support: Parser#append.

Due to (probably) Ruby's optimizations, `str1 << str2` could change
str1's encoding if str1 was blank.  It is critical to have @data in pure
ASCII and never allow UTF-8 there.
Merge pull request #3 from skalee/ruby-2-support

Ruby 2.0.0 support: Parser#append.