Revert "Use [] shortcut"

This reverts commit 4f292e71e9191a6f15f993ad1942c5de3957159a.
"Writing arbitrary attributes on a model is deprecated. Please just use `attr_writer` etc. (called from block (2 levels) in archive! at /Users/mkyed/Documents/src/firmafon/archiving/lib/archiving/archive_table.rb:108)"
Revert "Revert "Use [] shortcut""

This reverts commit d68c09b3c2fbf6128ed8c8327607df80d897f222.
"pryability" added
Test polymorphic sub archiving