Tweaks and more tests for maps

- Change the default message hash code to 1 to be consistent with other code
- Change the empty list/map hash code to 0 as "empty map" is equivalent to "no map"
- Removed map fields from unittest_proto3.proto
- Created map_unittest_proto3.proto which is like map_unittest.proto but proto3-only
- Fixed factory methods in FieldCodec highlighted by using all field types :)
- Added tests for map serialization:
  - Extra fields within entries
  - Entries with value then key
  - Non-contiguous entries for the same map
  - Multiple entries for the same key

Changes to generated code coming in next commit

2 files changed, 2 insertions, 3 deletions

diff --git a/csharp/src/ProtocolBuffers/Collections/MapField.cs b/csharp/src/ProtocolBuffers/Collections/MapField.cs
index 6834cfbb..9eed833f 100644
--- a/csharp/src/ProtocolBuffers/Collections/MapField.cs
+++ b/csharp/src/ProtocolBuffers/Collections/MapField.cs
@@ -261,7 +261,7 @@ namespace Google.Protobuf.Collections
         public override int GetHashCode()
         {
             var valueComparer = EqualityComparer<TValue>.Default;
-            int hash = 19;
+            int hash = 0;
             foreach (var pair in list)
             {
                 hash ^= pair.Key.GetHashCode() * 31 + valueComparer.GetHashCode(pair.Value);
@@ -380,7 +380,6 @@ namespace Google.Protobuf.Collections
                 private readonly Codec codec;
                 internal TKey Key { get; set; }
                 internal TValue Value { get; set; }
-                internal int Size { get; set; }
 
                 internal MessageAdapter(Codec codec)
                 {
diff --git a/csharp/src/ProtocolBuffers/Collections/RepeatedField.cs b/csharp/src/ProtocolBuffers/Collections/RepeatedField.cs
index ebfc522f..4d4212cb 100644
--- a/csharp/src/ProtocolBuffers/Collections/RepeatedField.cs
+++ b/csharp/src/ProtocolBuffers/Collections/RepeatedField.cs
@@ -193,7 +193,7 @@ namespace Google.Protobuf.Collections
 
         public override int GetHashCode()
         {
-            int hash = 23;
+            int hash = 0;
             for (int i = 0; i < count; i++)
             {
                 hash = hash * 31 + array[i].GetHashCode();