TZ display name changes no longer reset adjustment rules.

Reset of adjustment rules is mandatory when the base offset is changed because the rules are relative to the base offset.
Adjustment rules are now kept until the base offset changes.

mcs/class/System.Core/System/TimeZoneInfo.cs

@@ -1179,9 +1179,9 @@ namespace System
 				DateTime ttime = pair.Key;
 				TimeType ttype = pair.Value;
 				if (!ttype.IsDst) {
-					if (standardDisplayName != ttype.Name || baseUtcOffset.TotalSeconds != ttype.Offset) {
+					if (standardDisplayName != ttype.Name)
 						standardDisplayName = ttype.Name;
-						daylightDisplayName = null;
+					if (baseUtcOffset.TotalSeconds != ttype.Offset) {
 						baseUtcOffset = new TimeSpan (0, 0, ttype.Offset);
 						if (adjustmentRules.Count > 0) // We ignore AdjustmentRules but store transitions.
 							storeTransition = true;
@@ -1217,10 +1217,11 @@ namespace System
 					}
 					dst_observed = false;
 				} else {
-					if (daylightDisplayName != ttype.Name || dstDelta.TotalSeconds != ttype.Offset - baseUtcOffset.TotalSeconds) {
+					if (daylightDisplayName != ttype.Name)
 						daylightDisplayName = ttype.Name;
+					if (dstDelta.TotalSeconds != ttype.Offset - baseUtcOffset.TotalSeconds)
 						dstDelta = new TimeSpan(0, 0, ttype.Offset) - baseUtcOffset;
-					}
+
 					dst_start = ttime;
 					dst_observed = true;
 				}