adds spec for Bug [#17030]

[moofkit]

Solving #823

Enumerable#grep and Enumerable#grep_v when passed a Regexp and no block no longer modify
Regexp.last_match

There are specs already for $& - an global alias to Regexp.last_match. Maybe it make sense to leave one of them but use Regexp.last_match explicitly because it seems more clear IMO.

[eregon]

Regexp.last_match and $& are the same thing, and given $& existed before I'd see Regexp.last_match as the alias, but I guess it's subjective.

I'm not really against adding these extra specs, so I'll merge this, though I wouldn't do that in too many places because it just checks again that they are the same thing.