move method_missing impl into our OpenStruct sub-class to avoid clash on Ruby 2.3
move method_missing impl into our OpenStruct sub-class to avoid clash on Ruby 2.3

ostruct.rb changed a bit on 2.3 but this makes it so that is behaves nicely on 2.2/2.3

this is expected to fix #366
fedenusy/warbler,fedenusy/warbler,fedenusy/warbler,fedenusy/warbler