add test case for string argument

Author: t-mangoe

test/test_timeout.rb

@@ -54,6 +54,12 @@ def test_raise_for_neg_second
     end
   end
 
+  def test_raise_for_string_argument
+    assert_raise(NoMethodError) do
+      Timeout.timeout("1") { sleep(0.01) }
+    end
+  end
+
   def test_included
     c = Class.new do
       include Timeout