* string.c (fstring_cmp): take string encoding into account when
  comparing fstrings [ruby-core:57037] [Bug #8866]

* test/ruby/test_string.rb: add test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42847 b2dd03c8-39d4-4d8f-98ff-823fe69b080e