Fixing test by removing redundant ToString

test/Microsoft.Extensions.Caching.Memory.Tests/TimeExpirationTests.cs

@@ -37,7 +37,7 @@ namespace Microsoft.Extensions.Caching.Memory
             },
             nameof(MemoryCacheEntryOptions.AbsoluteExpiration),
             "The absolute expiration value must be in the future.",
-            expected.ToString(CultureInfo.CurrentCulture));
+            expected);
         }
 
         [Fact]