update metadata

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

response.go

@@ -2,10 +2,11 @@ package grpc
 
 import (
 	"github.com/unistack-org/micro/v3/codec"
+	"github.com/unistack-org/micro/v3/metadata"
 )
 
 type rpcResponse struct {
-	header map[string]string
+	header metadata.Metadata
 	codec  codec.Codec
 }
 
@@ -13,7 +14,7 @@ func (r *rpcResponse) Codec() codec.Writer {
 	return r.codec
 }
 
-func (r *rpcResponse) WriteHeader(hdr map[string]string) {
+func (r *rpcResponse) WriteHeader(hdr metadata.Metadata) {
 	for k, v := range hdr {
 		r.header[k] = v
 	}