add refute_nil to ActiveSupport::TestCase

add refute_nil to ActiveSupport::TestCase

ActiveSupport::TestCase extends Test::Unit::TestCase in Ruby 1.8,
whereas in Ruby 1.9, Test::Unit is implemented by MiniTest under the
hood. This means that ActiveSupport::TestCase in Ruby 1.8 is missing
some functions that minitest from Ruby 1.9 provides.

refute_nil is one of those missing functions in Ruby 1.8. Add it in our
test_helper.

This fixes various test suite failures errors about refute_nil being an
undefined method.