Fix bug in filter code that caused Rubinius failures

Although this was fixed in commit bc7c532c5 (by forcing the values to be
Strings if they are Symbols), this commit fixes the underlying problem.

The real bug was in allowing the Symbol :all to remain in the Regexp
list when it is not needed. The :all and :none modes don't use a Regexp
and so @rex is now set to nil in those cases.
Fix bug in filter code that caused Rubinius failures

Although this was fixed in commit bc7c532c5 (by forcing the values to be
Strings if they are Symbols), this commit fixes the underlying problem.

The real bug was in allowing the Symbol :all to remain in the Regexp
list when it is not needed. The :all and :none modes don't use a Regexp
and so @rex is now set to nil in those cases.
Merge remote-tracking branch 'origin/filter_fix_regex_symbol' into filter_fix_regex_symbol
Merge pull request #1191 from townsen/filter_fix_regex_symbol

Fix bug in filter code that caused Rubinius failures