Raise exception on Cache.clear call with string argument

Options argument from `clear(options)` in all bundled cache implementations
wasn't used at all, so calling `clear('some_string')` just flushes cache
without any warning. But this method can be easily confused with `delete`,
exactly what happened on my project. So I think it's better to check
if `options` is a hash, when it provided, and raise ArgumentError if not.