Refactor default_value to use if-let
Refactor default_value to use if-let

thorbenk/serde,skade/serde,Hoverbear/serde,erickt/serde,csmulhern/serde,serde-rs/serde,3Hren/serde,JohnHeitmann/serde,Ticki/serde,oli-obk/rust-serde,eefriedman/serde,Byron/serde,jmesmon/serde,tempbottle/serde,sidred10/serde,infinityb/serde,pcwalton/serde