Fix all matcher exception when actual not enumerable

When the action value of passed to the `all` matcher is not enumerable,
rather than failing to match and returning an error it raises an
exception. This commit ensures that `actual` is enumerable before
proceeding with matching.
I was following the pattern here (although in this case no logic is necessary):

https://github.com/rspec/rspec-expectations/blob/a72203a0384487c067bad5133e13a6b706803b63/lib/rspec/matchers/built_in/be_between.rb#L67-L69