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 unsigned_tx = "...";
    const std::string prefixed_message =
        std::string(1, (char)MESSAGE_MAGIC.length()) +
        MESSAGE_MAGIC +
        std::string(1, (char)unsigned_tx.length()) +
        unsigned_tx;
    const uint256 signature_hash = Hash(unsigned_tx);
    const uint256 message_hash1 = Hash(prefixed_message);
    const uint256 message_hash2 = MessageHash(unsigned_tx);
    BOOST_CHECK_EQUAL(message_hash1, message_hash2);
    BOOST_CHECK_NE(message_hash1, signature_hash);
}
BOOST_AUTO_TEST_CASE(remove_prefix)
{
    BOOST_CHECK_EQUAL(RemovePrefix("./util/system.h", "./"), "util/system.h");
    BOOST_CHECK_EQUAL(RemovePrefix("foo", "foo"), "");
    BOOST_CHECK_EQUAL(RemovePrefix("foo", "fo"), "o");
    BOOST_CHECK_EQUAL(RemovePrefix("foo", "f"), "oo");
    BOOST_CHECK_EQUAL(RemovePrefix("foo", ""), "foo");
    BOOST_CHECK_EQUAL(RemovePrefix("fo", "foo"), "fo");
    BOOST_CHECK_EQUAL(RemovePrefix("f", "foo"), "f");
    BOOST_CHECK_EQUAL(RemovePrefix("", "foo"), "");
    BOOST_CHECK_EQUAL(RemovePrefix("", ""), "");
}
BOOST_AUTO_TEST_SUITE_END()