use stdlib header instead

middleware/header.go

 import "net/http"
 
 // Header adds the given response headers to the upcoming `http.ResponseWriter`.
-func Header(values map[string]string) func(http.Handler) http.Handler {
+func Header(values http.Header) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			for key, value := range values {
-				w.Header().Set(key, value)
+				w.Header()[key] = value
 			}
 			next.ServeHTTP(w, r)
 		})

middleware/header_test.go

 )
 
 func TestHeader(t *testing.T) {
-	var values = make(map[string]string)
-	values["X-Powered-By"] = "rex server"
+	var values = make(http.Header)
+	values.Set("X-Powered-By", "rex")
 
 	app := rex.New()
 	app.Use(Header(values))
 
 		app.ServeHTTP(response, request)
 
-		So(response.Header().Get("X-Powered-By"), ShouldEqual, "rex server")
+		So(response.Header().Get("X-Powered-By"), ShouldEqual, "rex")
 	})
 }