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

const std::string& section,
        const std::string& name,
        const std::string& value_prefix)
    {
        std::vector<std::string> values;
        int suffix = 0;
        for (Action action : actions) {
            if (action == NONE) break;
            std::string prefix;
            if (action == SECTION_SET || action == SECTION_NEGATE) prefix = section + ".";
            if (action == SET || action == SECTION_SET) {
                for (int i = 0; i < 2; ++i) {
                    values.push_back(prefix + name + "=" + value_prefix + ToString(++suffix));
                }
            }
            if (action == NEGATE || action == SECTION_NEGATE) {
                values.push_back(prefix + "no" + name + "=1");
            }
        }
        return values;
    }
};
BOOST_FIXTURE_TEST_CASE(util_ArgsMerge, ArgsMergeTestingSetup)