Fix normalization of partial messages

[timostamm]

Connect supports two styles of passing a request message to a client.

1. With a concrete instance:

const req = new SayRequest({ sentence: "hello" });
client.say(req);

2. Or with an object literal:

client.say({ sentence: "hello" });

The object literal is just a convenient shortcut, and simply calls new SayRequest with the given partial message as an argument.

There is a flaw in the logic that converts a partial message to a concrete instance: If a different message instance is passed to the client, it is not converted to the correct instance. For example, provided that the Protobuf message SimilarToSayRequest also has a field string sentence like the original, the following code compiles without error:

const req = new SimilarToSayRequest({ sentence: "hello" });
client.say(req);

Note that TypeScript does not use nominal typing, and passing a different message is not a type error if it structurally matches. The issue is that the message properties are not converted via new SayRequest, and the serialization logic of SimilarToSayRequest is applied, which will lead to corrupt data for the RPC unless the same Protobuf field numbers are used.

We recommend to stick with one of the two styles above, and not pass different message types to an RPC, but it's only logical to treat a different message type the same way as we treat an object literal.

This PR changes the behavior to convert the request message via new SayRequest, unless the message is an instance of SayRequest (required to retain unknown fields).