refactor: change test method to GetValueAsync.

Author: ikesnowy

Enyim.Caching.Tests/MemcachedClientGetTests.cs

@@ -117,8 +117,8 @@ public async Task When_Getting_Empty_String_Async_Result_Is_Successful()
             var key = GetUniqueKey("Get");
             var empty = string.Empty;
             Store(key: key, value: empty);
-            var getResult = await _client.GetAsync<string>(key);
-            Assert.Equal(getResult.Value, empty);
+            var getResult = await _client.GetValueAsync<string>(key);
+            Assert.Equal(getResult, empty);
         }
 
         [Fact]