Fix #toggle on dimmer items (#378)

Signed-off-by: Cody Cutrer <cody@cutrer.us>

lib/openhab/core/items/switch_item.rb

@@ -50,9 +50,7 @@ def format_type(command)
         # @return [self]
         #
         def toggle(source: nil)
-          return on!(source: source) unless state?
-
-          command!(!state, source: source)
+          command!(!on?, source: source)
         end
 
         # @!method on?

spec/openhab/core/items/dimmer_item_spec.rb

@@ -89,6 +89,28 @@
     end
   end
 
+  describe "#toggle" do
+    specify do
+      item.on.toggle
+      expect(item.state).to eq 0
+    end
+
+    specify do
+      item.off.toggle
+      expect(item.state).to eq 100
+    end
+
+    specify do
+      item.update(UNDEF).toggle
+      expect(item.state).to eq 100
+    end
+
+    specify do
+      item.update(NULL).toggle
+      expect(item.state).to eq 100
+    end
+  end
+
   it "works with grep" do
     items.build { switch_item "Switch1" }
     expect(items.grep(DimmerItem)).to match_array [DimmerOne, DimmerTwo]