Fix verified doubles when their const has already been stubbed.

Fixes #823.
Remove unnecessary captured block.

Capturing blocks isn’t free, and `yield` (which
we are already using) performs better than
capturing the block and using `block.call`.
Fix memoization to properly memoize nil.
Merge pull request #824 from rspec/fix-instance_double-when-class-const-stubbed

Fix verified doubles when their const has already been stubbed.