Fix splitting of empty strings on regex with trim: true

Make String.split and Regex.split called on empty string with
trim: true return []. Add testcase for String.split, fix incorrect
testcase for Regex.split.
Fix string splitting on regex when both :trim and other options are present
Merge pull request #2906 from eugmes/empty-string-split-fix

Fix splitting of empty strings on regex with trim: true
Merge branch 'master' of https://github.com/elixir-lang/elixir