Ensure factorial is calculated for 0 (#102)

Ensure a unique solution for res0. Without the check, both answers 0 and 1 pass the test.

Co-authored-by: James Santucci <[email protected]>

Author: xiaowei-zhang

src/main/scala/scalatutorial/sections/TailRecursion.scala

@@ -145,6 +145,7 @@ object TailRecursion extends ScalaTutorialSection {
       iter(n, res1)
     }
 
+    factorial(0) shouldBe 1
     factorial(3) shouldBe 6
     factorial(4) shouldBe 24
   }