values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

{
    return std::string(span.begin(), span.end());
}
BOOST_AUTO_TEST_CASE(test_spanparsing)
{
    using namespace spanparsing;
    std::string input;
    Span<const char> sp;
    bool success;
    
    input = "MilkToastHoney";
    sp = input;
    success = Const("", sp); 
    BOOST_CHECK(success);
    BOOST_CHECK_EQUAL(SpanToStr(sp), "MilkToastHoney");
    success = Const("Milk", sp);
    BOOST_CHECK(success);
    BOOST_CHECK_EQUAL(SpanToStr(sp), "ToastHoney");
    success = Const("Bread", sp);
    BOOST_CHECK(!success);
    success = Const("Toast", sp);
    BOOST_CHECK(success);
    BOOST_CHECK_EQUAL(SpanToStr(sp), "Honey");
    success = Const("Honeybadger", sp);
    BOOST_CHECK(!success);
    success = Const("Honey", sp);
    BOOST_CHECK(success);