Fixes #212

This is a quick and dirty fix of issue #212.  
Partially reverts previous changes from commit 5fbd1f08c4
Re-implements the change in a slightly different way.

lnbits/extensions/lnurlp/lnurl.py

@@ -95,13 +95,17 @@ async def api_lnurl_callback(link_id):
         extra={"tag": "lnurlp", "link": link.id, "comment": comment},
     )
 
-    resp = {
-        "routes": [],
-        "pr": payment_request,
-    }
-
     success_action = link.success_action(payment_hash)
     if success_action:
-        resp["success_action"] = success_action
+        resp = LnurlPayActionResponse(
+            pr=payment_request,
+            success_action=success_action,
+            routes=[],
+        )
+    else:
+        resp = LnurlPayActionResponse(
+            pr=payment_request,
+            routes=[],
+        )
 
-    return jsonify(resp), HTTPStatus.OK
+    return jsonify(resp.dict()), HTTPStatus.OK