Fix multiple header writes for connect unary on error (#726)

On error a connect unary handler will call `Close(err)` to send the
final error. For non-nil errors this will always attempt to encode the
header status by calling `WriteHeader`. If the body has already been
written this triggers the following log via `http.Server`:
```
http: superfluous response.WriteHeader call from connectrpc.com/connect.(*connectUnaryHandlerConn).Close (protocol_connect.go:743)
```
The common case for this superfluous log is when a message send is
interrupted, due to a context cancel or other write error, and the error
is then attempting to re-encode the headers and body.

This PR now moves the `wroteBody` check to a `wroteHeader` check on the
unmarshaller. When any payload is sent, including a nil field for header
only, we now stop attempting to encode any following errors, as it would
be superfluous.

connect.go

@@ -431,9 +431,10 @@ func receiveUnaryMessage[T any](conn receiveConn, initializer maybeInitializer,
 	if err := initializer.maybe(conn.Spec(), &msg2); err != nil {
 		return nil, err
 	}
-	if err := conn.Receive(&msg2); err == nil {
-		return nil, NewError(CodeUnimplemented, fmt.Errorf("unary %s has multiple messages", what))
-	} else if err != nil && !errors.Is(err, io.EOF) {
+	if err := conn.Receive(&msg2); !errors.Is(err, io.EOF) {
+		if err == nil {
+			err = NewError(CodeUnimplemented, fmt.Errorf("unary %s has multiple messages", what))
+		}
 		return nil, err
 	}
 	return &msg, nil

protocol_connect.go

@@ -686,7 +686,6 @@ type connectUnaryHandlerConn struct {
 	marshaler       connectUnaryMarshaler
 	unmarshaler     connectUnaryUnmarshaler
 	responseTrailer http.Header
-	wroteBody       bool
 }
 
 func (hc *connectUnaryHandlerConn) Spec() Spec {
@@ -709,8 +708,7 @@ func (hc *connectUnaryHandlerConn) RequestHeader() http.Header {
 }
 
 func (hc *connectUnaryHandlerConn) Send(msg any) error {
-	hc.wroteBody = true
-	hc.writeResponseHeader(nil /* error */)
+	hc.mergeResponseHeader(nil /* error */)
 	if err := hc.marshaler.Marshal(msg); err != nil {
 		return err
 	}
@@ -726,16 +724,16 @@ func (hc *connectUnaryHandlerConn) ResponseTrailer() http.Header {
 }
 
 func (hc *connectUnaryHandlerConn) Close(err error) error {
-	if !hc.wroteBody {
-		hc.writeResponseHeader(err)
+	if !hc.marshaler.wroteHeader {
+		hc.mergeResponseHeader(err)
 		// If the handler received a GET request and the resource hasn't changed,
 		// return a 304.
 		if len(hc.peer.Query) > 0 && IsNotModifiedError(err) {
 			hc.responseWriter.WriteHeader(http.StatusNotModified)
 			return hc.request.Body.Close()
 		}
 	}
-	if err == nil {
+	if err == nil || hc.marshaler.wroteHeader {
 		return hc.request.Body.Close()
 	}
 	// In unary Connect, errors always use application/json.
@@ -757,7 +755,7 @@ func (hc *connectUnaryHandlerConn) getHTTPMethod() string {
 	return hc.request.Method
 }
 
-func (hc *connectUnaryHandlerConn) writeResponseHeader(err error) {
+func (hc *connectUnaryHandlerConn) mergeResponseHeader(err error) {
 	header := hc.responseWriter.Header()
 	if hc.request.Method == http.MethodGet {
 		// The response content varies depending on the compression that the client
@@ -923,6 +921,7 @@ type connectUnaryMarshaler struct {
 	bufferPool       *bufferPool
 	header           http.Header
 	sendMaxBytes     int
+	wroteHeader      bool
 }
 
 func (m *connectUnaryMarshaler) Marshal(message any) *Error {
@@ -961,6 +960,7 @@ func (m *connectUnaryMarshaler) Marshal(message any) *Error {
 }
 
 func (m *connectUnaryMarshaler) write(data []byte) *Error {
+	m.wroteHeader = true
 	payload := bytes.NewReader(data)
 	if _, err := m.sender.Send(payload); err != nil {
 		err = wrapIfContextError(err)