Add zeroth prime test case. Fixes #231

exercises/nth-prime/example.scala

@@ -4,5 +4,7 @@ object Prime {
   private lazy val primes: Stream[BigInt] =
     Stream.cons(BigInt(2), primes.map(b => new BigInt(b.bigInteger.nextProbablePrime)))
 
-  def nth(n: Int): Int = primes.drop(n - 1).head.toInt
+  def nth(n: Int): Option[Int] =
+    if (n < 1) None
+    else Some(primes.drop(n - 1).head.toInt)
 }

exercises/nth-prime/src/test/scala/PrimeTest.scala

@@ -2,31 +2,36 @@ import org.scalatest.{Matchers, FlatSpec}
 
 class PrimeTest extends FlatSpec with Matchers {
   it should "calculate 1st prime" in {
-    Prime.nth(1) should be (2)
+    Prime.nth(1) should be (Some(2))
   }
 
   it should "calculate 2nd prime" in {
     pending
-    Prime.nth(2) should be (3)
+    Prime.nth(2) should be (Some(3))
   }
 
   it should "calculate 6th prime" in {
     pending
-    Prime.nth(6) should be (13)
+    Prime.nth(6) should be (Some(13))
   }
 
   it should "calculate 1000th prime" in {
     pending
-    Prime.nth(1000) should be (7919)
+    Prime.nth(1000) should be (Some(7919))
   }
 
   it should "calculate 10000th prime" in {
     pending
-    Prime.nth(10000) should be (104729)
+    Prime.nth(10000) should be (Some(104729))
   }
 
   it should "calculate 10001th prime" in {
     pending
-    Prime.nth(10001) should be (104743)
+    Prime.nth(10001) should be (Some(104743))
+  }
+
+  it should "not find zeroth prime" in {
+    pending
+    Prime.nth(0) should be (None)
   }
 }