Dispose correctly the cancellation token

Author: TimLariviere

src/Fabulous.Tests/CmdTests.fs

@@ -31,4 +31,13 @@ type ``Cmd tests``() =
             do! Async.Sleep 125
 
             Assert.AreEqual(Some(NewValue 3), actualValue)
+
+            actualValue <- None
+
+            triggerCmd 4 |> CmdTestsHelper.execute dispatch
+            do! Async.Sleep 75
+            triggerCmd 5 |> CmdTestsHelper.execute dispatch
+            do! Async.Sleep 125
+
+            Assert.AreEqual(Some(NewValue 5), actualValue)
         }

src/Fabulous/Cmd.fs

@@ -122,6 +122,9 @@ module Cmd =
                       async {
                           do! Async.Sleep(timeout)
                           dispatch(fn value)
+
+                          cts.Dispose()
+                          cts <- null
                       },
                       cts.Token
                   ) ]