serialization: do not write optional fields with default value

contrib/epee/include/serialization/keyvalue_serialization.h

@@ -89,6 +89,8 @@ public: \
 
 #define KV_SERIALIZE_OPT_N(variable, val_name, default_value) \
   do { \
+    if (is_store && this_ref.variable == default_value) \
+      break; \
     if (!epee::serialization::selector<is_store>::serialize(this_ref.variable, stg, hparent_section, val_name)) \
       epee::serialize_default(this_ref.variable, default_value); \
   } while (0);

tests/functional_tests/transfer.py

@@ -169,7 +169,7 @@ class TransferTest():
         assert e.subaddr_indices == [{'major': 0, 'minor': 0}]
         assert e.address == '42ey1afDFnn4886T7196doS9GPMzexD9gXpsZJDwVjeRVdFCSoHnv7KPbBeGpzJBzHRCAs9UxqeoyFQMYbqSWYTfJJQAWDm'
         assert e.double_spend_seen == False
-        assert e.confirmations == 0
+        assert not 'confirmations' in e or e.confirmations == 0
 
         running_balances[0] -= 1000000000000 + fee
 
@@ -282,7 +282,7 @@ class TransferTest():
         assert e.subaddr_indices == [{'major': 0, 'minor': 0}]
         assert e.address == '44Kbx4sJ7JDRDV5aAhLJzQCjDz2ViLRduE3ijDZu3osWKBjMGkV1XPk4pfDUMqt1Aiezvephdqm6YD19GKFD9ZcXVUTp6BW'
         assert e.double_spend_seen == False
-        assert e.confirmations == 0
+        assert not 'confirmations' in e or e.confirmations == 0
         assert e.amount == amount
         assert e.fee == fee