add missing tests for xml_mini and fix wrong argument error in date.to_s...

* added more tests for ActiveSupport ::XmlMini  to_tag method
* fix argument error in xml_mini.rb:50

XmlMiniTest::ToTagTest#test_#to_tag_accepts_date_types:
ArgumentError: wrong number of arguments (1 for 0)
    /Users/emre/code/rails/activesupport/lib/active_support/xml_mini.rb:50:in `to_s'