* wallet/rpc: Use the default maxfeerate value as BTC/kB
* test: Add test for default maxfeerate in sendrawtransaction
This patch adds test for the following case
- maxfeerate is omitted, and the actual fee rate is higher than
the default value(0.1 BTC/kB)
This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/16521 | PR16521]]