[BUG] cors middleware will add multiple headers when used with Party

https://github.com/iris-contrib/middleware/blob/master/cors/cors_test.go

only difference is that I use a Party

```go
...
app.UseRouter(crs)
...
v1 := app.Party("/v1").AllowMethods(iris.MethodOptions)
v1.UseRouter(crs) // same behavior for v1.Use(crs)
...
```