Fix description for failure when predicate returns nil
Fix respond_to? on ProxyClass
Don't raise NameErrors in `be` matcher
Add spec for memorization of exist matcher.

Follow up to #515.
Add copy logic to MatcherDelegator.
Clone matchers in composable matcher expressions.
Clone matchers in compound expressions as well.
Merge pull request #517 from rspec/add-memoization-spec-to-exist

Add spec for memorization of exist matcher.
Merge pull request #518 from rspec/clone-matchers-when-composing

Clone matchers when composing
Merge pull request #513 from danielfone/composable-be-matcher

Don't raise NameErrors in `be` matcher
Add changelog entry for #513.

[ci skip]