Make the #on_block method shorter
Make the #on_block method shorter

Rubocop was complaing that it was too long now. And the assignment
`@described_class = described_class` seems unnecessary, since that's the only
use of the local variable `described_class`.

backus/rubocop-rspec,nevir/rubocop-rspec,bquorning/rubocop-rspec,backus/rubocop-rspec