Make FieldDescriptor.IsPacked work appropriately.

This is a bit of a grotty hack, as we need to sort of fake proto2 field presence, but with only a proto3 version of the descriptor messages (a bit like oneof detection).
Should be okay, but will need to be careful of this if we ever implement proto2.

csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs

@@ -168,14 +168,16 @@ namespace Google.Protobuf.Reflection
             get { return fieldType == FieldType.Message && messageType.Proto.Options != null && messageType.Proto.Options.MapEntry; }
         }
 
-        // TODO(jonskeet): Check whether this is correct with proto3, where we default to packed...
-
         /// <summary>
         /// Returns <c>true</c> if this field is a packed, repeated field; <c>false</c> otherwise.
         /// </summary>
         public bool IsPacked
         {
-            get { return Proto.Options != null && Proto.Options.Packed; }
+            // Note the || rather than && here - we're effectively defaulting to packed, because that *is*
+            // the default in proto3, which is all we support. We may give the wrong result for the protos
+            // within descriptor.proto, but that's okay, as they're never exposed and we don't use IsPacked
+            // within the runtime.
+            get { return Proto.Options == null || Proto.Options.Packed; }
         }        
 
         /// <summary>

csharp/src/Google.Protobuf/Reflection/PartialClasses.cs

@@ -44,4 +44,16 @@ namespace Google.Protobuf.Reflection
             OneofIndex = -1;
         }
     }
+
+    internal partial class FieldOptions
+    {
+        // We can't tell the difference between "explicitly set to false" and "not set"
+        // in proto3, but we need to tell the difference for FieldDescriptor.IsPacked.
+        // This won't work if we ever need to support proto2, but at that point we'll be
+        // able to remove this hack and use field presence instead. 
+        partial void OnConstruction()
+        {
+            Packed = true;
+        }
+    }
 }