Fix LT-21976: Merge Create dates when merging entries (#334)

* Store the date at the start of the import and use it in entry
  creation. We need to have a consistent date recorded to determine
  if we read a date from the xml data or not.
* Add a function to return the newest date that was actually read
  from the xml data.
* Fix missing disposes in tests

Author: jasonleenaylor

src/SIL.LCModel/Application/ApplicationServices/XmlImportData.cs

@@ -165,6 +165,7 @@ internal string XmlState
 		private ReferenceTracker m_rglinks = new ReferenceTracker();
 		private TextWriter m_wrtrLog;
 		private bool m_createLinks;
+		private DateTime m_importDate = DateTime.Now;
 
 
 		/// ------------------------------------------------------------------------------------
@@ -470,6 +471,7 @@ private void MergeRedundantEntries(ILexEntry leOld, ILexEntry leNew)
 			ILexSense[] rgls = leOld.SensesOS.ToArray();
 			for (int i = 0; i < rgls.Length; ++i)
 				leNew.SensesOS.Add(rgls[i]);
+			MergeDates(leOld, leNew);
 			MergeEntryRefs(leOld, leNew);
 			CopyCustomFieldData(leOld, leNew);
 
@@ -479,6 +481,23 @@ private void MergeRedundantEntries(ILexEntry leOld, ILexEntry leNew)
 				m_mapIdGuid[id] = leNew.Guid;
 		}
 
+		/// Use the newest date between the two dates, as long as it is different from the importDate
+		private void MergeDates(ILexEntry leOld, ILexEntry leNew)
+		{
+			if (leOld.DateCreated == m_importDate)
+			{
+				return;
+			}
+			if (leNew.DateCreated == m_importDate)
+			{
+				leNew.DateCreated = leOld.DateCreated;
+			}
+			else if (leOld.DateCreated > leNew.DateCreated)
+			{
+				leNew.DateCreated = leOld.DateCreated;
+			}
+		}
+
 		private void MergeEntryRefs(ILexEntry leOld, ILexEntry leNew)
 		{
 			if (leOld.EntryRefsOS.Count == 0)
@@ -1089,6 +1108,7 @@ private void ReadXmlObject(XmlReader xrdr, FieldInfo fi, ICmObject objToUse)
 								if (m_factLexEntry == null)
 									m_factLexEntry = m_cache.ServiceLocator.GetInstance<ILexEntryFactory>();
 								cmo = m_factLexEntry.Create();
+								((LexEntry)cmo).DateCreated = m_importDate;
 								m_nHomograph = 0;
 								break;
 							case TextTags.kClassId: