Convert URLs to fully-qualified for HTTP redirects (#19020)

* Convert URLs to fully-qualified for HTTP redirects

* forgot the tests

web/saml.go

@@ -110,6 +110,7 @@ func completeSaml(c *Context, w http.ResponseWriter, r *http.Request) {
 		redirectURL = val
 		hasRedirectURL = val != ""
 	}
+	redirectURL = fullyQualifiedRedirectURL(c.GetSiteURLHeader(), redirectURL)
 
 	handleError := func(err *model.AppError) {
 		if isMobile && hasRedirectURL {
@@ -184,7 +185,6 @@ func completeSaml(c *Context, w http.ResponseWriter, r *http.Request) {
 			})
 			utils.RenderMobileAuthComplete(w, redirectURL)
 		} else {
-			redirectURL = c.GetSiteURLHeader() + redirectURL
 			http.Redirect(w, r, redirectURL, http.StatusFound)
 		}
 		return