Small patch to mysql_adapter for Ruby 1.9 #3156

Hi, I'd be keen for this little patch to mysql_adapter to make it into
2.3.5 - can anyone from core take it on?

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/3156

It's just a one line change to correctly test for a symbol in the
returned array from instance_methods (and also still testing for a
string as was the ruby 1.8 way).

Thank you

Stephen

(Why is it important? It breaks slim-attributes under ruby 1.9, see
http://slim-attributes.rubyforge.org/ )