add missing refactoring of json parser for calibre plugin
add missing refactoring of json parser for calibre plugin

NickSavage/koreader,koreader/koreader,frankyifei/koreader,noname007/koreader,mwoz123/koreader,ashhher3/koreader,pazos/koreader,chrox/koreader,Hzj-jie/koreader,apletnev/koreader,houqp/koreader,koreader/koreader,robert00s/koreader,NiLuJe/koreader,Frenzie/koreader,ashang/koreader,mihailim/koreader,Frenzie/koreader,poire-z/koreader,lgeek/koreader,poire-z/koreader,Markismus/koreader,NiLuJe/koreader,chihyang/koreader