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

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.
Merge pull request #711 from rspec/fix-710

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