Fix edge case that triggered “can't add a new key into hash during iteration"

Created by: myronmarston

This was triggered when a double was used as a with arg but was otherwise unused — in this case, there was no mock proxy for it, but while verifying one would get created (due to internal implementation details). Doing so in a each_value loop caused the error.

Fixes #710 (closed).

/cc @samphippen