Series: Tests seemed to imply that the return value must be a vector, but accept lists as well

[CrowsVeldt]

All but one of the tests check the return value of slices against a vector (the exception being number<string which checks against a set), at first I assumed this meant that the return value was supposed to be a vector specifically, but then I realized that all the tests will pass if given a list instead. I assume because they're both seqs?

Clearly this isn't a major issue, but I did find it quite confusing at first. Is the returned value supposed to be a vector, specifically? If so, maybe the tests should be changed. If not, than maybe it should be made explicit? Either by the tests being rewritten, or perhaps just by adding a note in the README.

[tasxatzial]

I believe this issue is similar to #229 opened by the same person.