Lack of grpc Trailers-Only support in the wrapHandler function

[JustCryingCat]

Versions of relevant software used
v0.15.0

What happened
The wrapHandler function does not check Trailers-Only case and adds extra data to trailers.

grpc-web/go/grpcweb/grpc_web_response.go

Lines 97 to 106 in 1d9bbb0

	func (w *grpcWebResponse) copyTrailersToPayload() {
	trailers := extractTrailingHeaders(w.headers, w.wrapped.Header())
	trailerBuffer := new(bytes.Buffer)
	trailers.Write(trailerBuffer)
	trailerGrpcDataHeader := []byte{1 << 7, 0, 0, 0, 0} // MSB=1 indicates this is a trailer data frame.
	binary.BigEndian.PutUint32(trailerGrpcDataHeader[1:5], uint32(trailerBuffer.Len()))
	w.wrapped.Write(trailerGrpcDataHeader)
	w.wrapped.Write(trailerBuffer.Bytes())
	flushWriter(w.wrapped)
	}

It's about line 101

Instead of empty trailers in the case of Trailers-Only, I get a trailer with 0x80 0x0 0x0 0x0 0x0

What you expected to happen
In the case of Trailers-Only, Trailer must remain empty. In the response from the server only headers are involved

How to reproduce it (as minimally and precisely as possible):
Your library uses the traefik gateway, which allows the case of Trailers-Only.

https://github.com/traefik/traefik/blob/8174860770e536b4afb541e0ab13b3611a101430/pkg/middlewares/grpcweb/grpcweb.go#L15-L26