NodeSet#each returns zero instead of self

[olehif]

According to ruby convention, #each method should return self:
[1, 2].each {} # => [1, 2]

While NodeSet#each always return zero instead:
nodes.each {} # => 0

As everyone in the ruby community expects #each to always return self (I myself had a hard time debugging seemingly perfect line of code) I think it would make sense to change this behavior even though it might potentially be a breaking change.

[flavorjones]

Hey! Thanks for submitting this. I agree it's unexpected behavior to not act like an Enumerable or Array class, which is what NodeSet is intended to behave like.

That said, I'll first (and this is intended to be humorous!) point out that this is not an obvious assumption for "everyone in the ruby community": see #1678 where some people have opined that at least one aspect of Enumerable's behavior is non-intuitive.

Second, I do want to point out that this is slightly more complicated than you've presented. The Array#each method returns an Enumerator object when there's no block presented. This PR doesn't address that behavior, and that's OK, I just want to point out that there's a complex set of behaviors that despite our best intentions may never get fully implemented in NodeSet.

Further evidence of this complexity is in #1677 where I honestly failed to completely understand the issue(s) being submitted.

In any case, this is an awesome, quick fix for what I consider to be a bug. Thank you! Will merge and this and it should be in the next release! Will give you a mention in the CHANGELOG.