Image metadata should not be stripped from thumbnails by default

As per the discussion [here](https://github.com/refinery/refinerycms/pull/2261#issuecomment-29126082), it makes sense for the default behaviour of `Refinery::Image#thumbnail` to be consistent with that of Dragonfly and ImageMagick, i.e. not stripping metadata from images by default. This avoids unexpected behaviour.