[Proposal]Add test with 0 element but not empty

[meian]

Proposal.

Add common test case for empty element but no emptyIterable.
The test checks for no panic.
Use test source like below.

itb := gcf.FromSlice([]int{1})
itb = gcf.Filter(itb, func(v int) bool { return false })

What benefit with the proposal?

Since each Iterator does not expect to pass 0 Iterators, it could panic trying to process 0 Iterators.
By passing this test prevent that panic.

Others

emptyじゃない時にパニック起こさない汎用のテストケース作ってソース以外の関数に適用する

Originally posted by @meian in #83 (comment)