FieldMask.Merge not worked with WellKnownTypes wrappers

[heggi]

What version of protobuf and what language are you using?
Version: 3.19.1
Language: C#

What operating system (Linux, Windows, ...) and version?
Any OS (Try on Ubuntu 20.04, Windows 7, 10)

What runtime / compiler are you using (e.g., python version or gcc version)
.net5.0, .net6.0

What did you do?
Example code

syntax = "proto3";

import "google/protobuf/wrappers.proto";
import "google/protobuf/field_mask.proto";

option csharp_namespace = "Test.Proto";

message MergeDataRequest {
    google.protobuf.StringValue test = 1;
}

var mask = new Google.Protobuf.WellKnownTypes.FieldMask();
mask.Paths.Add("test");

var m1 = new Test.Proto.MergeDataRequest {
    Test = "testString",
}; 

var m2 = new Test.Proto.MergeDataRequest();
mask.Merge(m1, m2);

What did you expect to see
Successful merge from source to destination.

What did you see instead?

Last line got

Unhandled exception. System.InvalidCastException: Unable to cast object of type 'System.String' to type 'Google.Protobuf.IMessage'.
   at Google.Protobuf.FieldMaskTree.Merge(Node node, String path, IMessage source, IMessage destination, MergeOptions options) in /home/heggi/Projects/Google.Protobuf/FieldMaskTree.cs:line 336
   at Google.Protobuf.FieldMaskTree.Merge(IMessage source, IMessage destination, MergeOptions options) in /home/heggi/Projects/Google.Protobuf/FieldMaskTree.cs:line 243
   at Google.Protobuf.WellKnownTypes.FieldMask.Merge(IMessage source, IMessage destination, MergeOptions options) in /home/heggi/Projects/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs:line 324
   at Google.Protobuf.WellKnownTypes.FieldMask.Merge(IMessage source, IMessage destination) in /home/heggi/Projects/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs:line 332
   at Program.<Main>$(String[] args) in /home/heggi/Projects/test/Program.cs:line 9

Anything else we should know about your project / environment

Same trouble with any wrappers types

[boukeversteegh]

Thanks for reporting. Same issue here.