cypriss/mutations

 - Document default behavior discarding of nils for optional params
 - document string.discard_empty

 - protected parameters:
     optional do
       boolean :skip_confirmation, protected: true
     end
     Given the above, skip_confirmation is only accepted as a parameter if it's passed in a later hash, eg this would make it take:
     User::ChangeEmail.run!(params, user: current_user, skip_confirmation: true)
     But this would not:
     params = {user: current_user, skip_confirmation: true}
     User::ChangeEmail.run!(params)