Default to top_of_list value in the list plugin

This will make the list plugin honor the configured :top option
when the first item in the list is added when the position value
is not provided.

lib/sequel/plugins/list.rb

@@ -185,10 +185,13 @@ def prev(n = 1)
         end
 
         # Set the value of the position_field to the maximum value plus 1 unless the
-        # position field already has a value.
+        # position field already has a value. If the list is empty, the position will
+        # be set to the model's +top_of_list+ value.
         def before_validation
           unless get_column_value(position_field)
-            set_column_value("#{position_field}=", list_dataset.max(position_field).to_i+1)
+            current_max = list_dataset.max(position_field)
+            value = current_max.nil? ? model.top_of_list : current_max.to_i + 1
+            set_column_value("#{position_field}=", value)
           end
           super
         end

spec/extensions/list_spec.rb

@@ -113,6 +113,18 @@ def klass(opts={})
       "SELECT * FROM items WHERE (id = 3) ORDER BY scope_id, position LIMIT 1"]
   end
 
+  it "should have position field set to configured top value when creating first item in the list if not already set" do
+    @tc.dataset = @tc.dataset.with_autoid(1).with_fetch([[{:pos=>nil}], [{:id=>1, :position=>0}], [{:pos=>0}], [{:id=>2, :position=>1}]])
+    @tc.create.values.must_equal(:id=>1, :position=>0)
+    @tc.create.values.must_equal(:id=>2, :position=>1)
+    @db.sqls.must_equal ["SELECT max(position) AS max FROM items LIMIT 1",
+      "INSERT INTO items (position) VALUES (0)",
+      "SELECT * FROM items WHERE (id = 1) ORDER BY position LIMIT 1",
+      "SELECT max(position) AS max FROM items LIMIT 1",
+      "INSERT INTO items (position) VALUES (1)",
+      "SELECT * FROM items WHERE (id = 2) ORDER BY position LIMIT 1"]
+  end
+
   it "should update positions automatically on deletion" do
     @o.destroy
     @db.sqls.must_equal ["DELETE FROM items WHERE (id = 7)", "UPDATE items SET position = (position - 1) WHERE (position > 3)"]