Use define_method to support aliases with dashes.

[This change](https://github.com/fog/fog-core/blob/68524a22fde9fed70e61534532f3679d26ee451c/lib/fog/core/attributes/default.rb#L57-L70) that was added about an hour ago in 68524a22fde9fed70e61534532f3679d26ee451c breaks the `Storage` service in `fog-softlayer` which using HTTP headers (Content-Type, Content-Length, etc) as aliases.

This PR fixes that.