Added unit test to check "GetOrAdd" atomicness

UnitTests/UnitTests.cs

@@ -73,7 +73,7 @@ public async Task TestGetOrAdd()
 		}
 
 		[TestMethod]
-		public async Task TestAtomicness()
+		public async Task TestTryAddAtomicness()
 		{
 			int i = 0;
 
@@ -90,6 +90,28 @@ await TestHelper.RunConcurrently(20, () => {
 			Assert.IsTrue(i == 1, i.ToString());
 		}
 
+		[TestMethod]
+		public async Task TestGetOrAddAtomicNess()
+		{
+			int i = 0;
+
+			var cache = new FastCache<int, int>();
+
+			cache.GetOrAdd(42, k => {
+				i++;
+				return 1024;
+			}, TimeSpan.FromMilliseconds(100));
+
+
+			await TestHelper.RunConcurrently(20, () => {
+				if (cache.TryAdd(42, 42, TimeSpan.FromSeconds(1)))
+					i++;
+			});
+
+			//test that add factory was called only once
+			Assert.IsTrue(i == 1, i.ToString());
+		}
+
 		[TestMethod]
 		public async Task Enumerator()
 		{