Wrote RequestCleanText return with end-token test unit test.

TestCentre/UnitTests.fs

@@ -136,6 +136,14 @@
                 |> Async.RunSynchronously
             Assert.NotEmpty result
 
+        [<Fact>]
+        let ``RequestCleanText returns a string which ends with the correct end-token`` () =
+            let result =
+                Pancake.RequestCleanTextAsync (ValidGibberishLevelInput()) (ValidSentencesInput())
+                |> Async.RunSynchronously
+                |> EndsAsIntended
+            Assert.True(result)
+
         [<Fact>]
         let ``RequestCleanText returns intended number of sentences`` () =
             let desiredSentencesCount = (ValidSentencesInput())