fix for StringScanner bug

[luikore]

require 'strscan'
ss = StringScanner.new "\n"
/./ =~ "\n" #=> 0
# but strscan's behavior is strange:
ss.scan /./ #=> nil

ss = StringScanner.new "a\n"
ss.scan /[.\n]/ #=> nil -- this group is interpreted as /\n/

[kzhamaji]

According to Ruby's manual, /./ doesn't match '\n' without using multi-line mode.