Make compatible_language_from follow the RFC 2616 selection algorithm

We need to look for the longest matching prefix rather than just the
first language with the same basic language code, otherwise if we have
both zh-CN and zh-TW available then we may select the wrong one as we
will return he first one we see that starts with zh even if the user
had said they preferred a specific variant.

lib/http_accept_language.rb

@@ -42,9 +42,8 @@ def preferred_language_from(array)
   #
   def compatible_language_from(array)
     user_preferred_languages.map do |x|
-      x = x.to_s.split("-")[0]
       array.find do |y|
-        y.to_s.split("-")[0] == x
+        y.to_s =~ /^#{Regexp.escape(x.to_s)}(-|$)/
       end
     end.compact.first
   end