Patch incompatible code with newest Rails release

[rubendinho]

The latest Rails Security patch requires that polymorphic paths be constructed with symbols, rather than keys https://github.com/rails/rails/issues/42157

Release info: https://weblog.rubyonrails.org/2021/3/26/marcel-upgrade-releases/

[dgmstuart]

@rubendinho yeah I noticed that. I've been trying to get Godmin to build on recent ruby/rails, but running into issues.

[dgmstuart]

Cut a new gem for this - thanks for the contribution: https://rubygems.org/gems/godmin/versions/2.1.0