Fix "ambiguous first argument" warning.
Switch from `include_examples` to `it_behaves_like`.

`include_examples` includes the group contents in the current
group, and we had some `lets` that were redefining each other,
producing these warnings:

/Users/myron/code/rspec-dev/repos/rspec-core/lib/rspec/core/memoized_helpers.rb:199: warning: method redefined; discarding old klass
/Users/myron/code/rspec-dev/repos/rspec-mocks/spec/rspec/mocks/matchers/receive_spec.rb:233: warning: previous definition of klass was here
/Users/myron/code/rspec-dev/repos/rspec-core/lib/rspec/core/memoized_helpers.rb:206: warning: method redefined; discarding old klass
/Users/myron/code/rspec-dev/repos/rspec-core/lib/rspec/core/memoized_helpers.rb:206: warning: previous definition of klass was here
/Users/myron/code/rspec-dev/repos/rspec-core/lib/rspec/core/memoized_helpers.rb:199: warning: method redefined; discarding old klass
/Users/myron/code/rspec-dev/repos/rspec-mocks/spec/rspec/mocks/matchers/receive_spec.rb:260: warning: previous definition of klass was here
/Users/myron/code/rspec-dev/repos/rspec-core/lib/rspec/core/memoized_helpers.rb:206: warning: method redefined; discarding old klass
/Users/myron/code/rspec-dev/repos/rspec-core/lib/rspec/core/memoized_helpers.rb:206: warning: previous definition of klass was here

`it_behaves_like` defines a sub-group that is isolated, and
prevents this warning from occurring.
Explicitly enable `should` mocks syntax since we use it here.
Merge pull request #455 from rspec/address-warnings

Address warnings