do not allow a minimal fee to not have a ticker

resolve #369

x/cash/staticfee.go

@@ -133,10 +133,13 @@ func (d FeeDecorator) extractFee(ctx weave.Context, tx weave.Tx, store weave.KVS
 	}
 
 	cmp := gconf.Coin(store, GconfMinimalFee)
-	// minimum has no currency -> accept everything
+	if cmp.IsZero() {
+		return finfo, nil
+	}
 	if cmp.Ticker == "" {
-		cmp.Ticker = fee.Ticker
+		return nil, errors.Wrap(coin.ErrInvalidCurrency, "no ticker")
 	}
+
 	if !fee.SameType(cmp) {
 		return nil, coin.ErrInvalidCurrency.Newf("%s vs fee %s", cmp.Ticker, fee.Ticker)
 

x/cash/staticfee_test.go

@@ -98,11 +98,18 @@ func TestFees(t *testing.T) {
 			min:       min,
 			expect:    errors.ErrUnauthorized.Is,
 		},
-		"can pay in any fee": {
+		"fee without an empty ticker is not accepted": {
 			signers:   []weave.Condition{perm},
 			initState: []orm.Object{must(WalletWith(perm.Address(), &cash))},
 			fee:       &FeeInfo{Fees: &min},
 			min:       coin.NewCoin(0, 1000, ""),
+			expect:    coin.ErrInvalidCurrency.Is,
+		},
+		"no fee (zero value) is acceptable": {
+			signers:   []weave.Condition{perm},
+			initState: []orm.Object{must(WalletWith(perm.Address(), &cash))},
+			fee:       &FeeInfo{Fees: coin.NewCoinp(0, 1, "FOO")},
+			min:       coin.NewCoin(0, 0, ""),
 			expect:    noErr,
 		},
 		"wrong currency checked": {