Add spec for Axiom::Types::Encodable.extended

* This is testing the side effects of using Object#extend with
  Axiom::Types::Encodable to make sure all the behaviour in the hook
  is covered fully.
Add spec for Axiom::Types::Encodable#finalize

* Adds specs to ensure both ascii compatible and incompatible
  encodings are handled properly.
Fix docs to be more precise
Add specs for Axiom::Types::ValueComparable
Add specs for Axiom::Types::LengthComparable
Upgrade flay and yardstick thresholds