Add a spec to String#index

when it returns the character index of a more complex multibyte character string after offset
(to add a spec that breaks on my silly bug).

Author: Lillian Zhang

spec/ruby/core/string/index_spec.rb

@@ -146,6 +146,7 @@
 
   it "returns the character index after offset" do
     "われわれ".index("わ", 1).should == 2
+    "ありがとうありがとう".index("が",3).should == 7
   end
 
   it "returns the character index after a partial first match" do