This seems to be the only place where a STR_AMOUNT is used for a sats
denominated fee amount. Many other places use the raw NUM type for a fee
amount, for example getblockstats and getblocktemplate. This doesn't
change the actual result of the RPC call.
The change is motivated by OpenRPC, where the field was previously given
a 'x-bitcoin-unit' tag. This usually describes a decimal amount, and may
be confusing for consumers applying this tag.