Merge pull request 'move set content-type in client publish' (#138) from devstigneev/micro-client-grpc:v3 into v3

Reviewed-on: #138

grpc.go

@@ -803,7 +803,6 @@ func (g *grpcClient) publish(ctx context.Context, ps []client.Message, opts ...c
 
 	for _, p := range ps {
 		md := metadata.Copy(omd)
-		md[metadata.HeaderContentType] = p.ContentType()
 		topic := p.Topic()
 		if len(exchange) > 0 {
 			topic = exchange
@@ -815,6 +814,8 @@ func (g *grpcClient) publish(ctx context.Context, ps []client.Message, opts ...c
 			md.Set(k, v)
 		}
 
+		md[metadata.HeaderContentType] = p.ContentType()
+
 		// passed in raw data
 		if d, ok := p.Payload().(*codec.Frame); ok {
 			body = d.Data