Ensured culture invariant timestamp comversion in unit tests

This will help avoid issues with timestamp conversion between culture settings. That ought not be an issue with the unit tests, but it's an easy fix to be safe.

Author: JeremyCaney

OnTopic.Data.Transfer.Tests/LastModifiedImportStrategyTest.cs

@@ -4,6 +4,7 @@
 | Project       Topics Library
 \=============================================================================================================================*/
 using System;
+using System.Globalization;
 using System.Linq;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using OnTopic.Attributes;
@@ -169,7 +170,7 @@ public void ImportAsInherit_TopicDataWithLastModified_SkipsExistingValue() {
 
       topic.Import(topicData);
 
-      Assert.AreEqual(oldTime.ToString(), topic.Attributes.GetValue("LastModified"));
+      Assert.AreEqual(oldTime.ToString(CultureInfo.InvariantCulture), topic.Attributes.GetValue("LastModified"));
 
     }
 
@@ -241,7 +242,7 @@ public void ImportAsTargetValue_TopicDataWithLastModified_SkipsExistingValue() {
         }
       );
 
-      Assert.AreEqual(yesterday.ToString(), topic.Attributes.GetValue("LastModified"));
+      Assert.AreEqual(yesterday.ToString(CultureInfo.InvariantCulture), topic.Attributes.GetValue("LastModified"));
 
     }
 
@@ -267,7 +268,7 @@ public void ImportAsSystem_TopicDataWithoutChanges_SkipsExistingValue() {
         }
       );
 
-      Assert.AreEqual(tomorrow.ToString(), topic.Attributes.GetValue("LastModified"));
+      Assert.AreEqual(tomorrow.ToString(CultureInfo.InvariantCulture), topic.Attributes.GetValue("LastModified"));
 
     }