Add specs on MatchData#begin with no match for index.
Add specs on MatchData#end with no match for index.
Add specs on MatchData#offset with no match for index.
MatchData#begin and MatchData#end return nil with no match for index.
Merge pull request #2447 from razielgn/match-data-offset

MatchData#begin and MatchData#end return nil with no match for index.