bolt12: fix LnAddr tag use in bolt12.to_lnaddr

Author: accumulator

electrum/bolt12.py

@@ -139,10 +139,10 @@ class WrappedBytesKey:
     fallbacks = data.get('invoice_fallbacks', [])
     fallbacks = list(filter(lambda x: x['version'] <= 16 and 2 <= len(x['address'] <= 40), fallbacks))
     if fallbacks:
-        addr.tags['f'] = fallbacks[0]
+        addr.tags.append(('f', fallbacks[0]))
     exp = data.get('invoice_relative_expiry', {}).get('seconds_from_creation', 0)
     if exp:
-        addr.tags['x'] = int(exp)
+        addr.tags.append(('x', int(exp)))
 
     return addr