Fix default value of `og:description` in `open_graph`
Fix default value of `og:description` in `open_graph`

By default, `og:description` should be `page.excerpt` or first 200 characters of `page.content`.

However, in the current implementation, `page.excerpt` will always be overwritten by `page.content`.

zhangg/hexo,beni55/hexo,luinnx/hexo,will-zhangweilin/myhexo,littledogboy/hexo,sundyxfan/hexo,runlevelsix/hexo,BruceChao/hexo,keeleys/hexo,SampleLiao/hexo,liukaijv/hexo,hugoxia/hexo,xiaoliuzi/hexo,XGHeaven/hexo,oomusou/hexo,allengaller/hexo,jp1017/hexo,hanhailong/hexo,jonesgithub/hexo,wflmax/hexo,delkyd/hexo,hackjustu/hexo,fuchao2012/hexo,wangjordy/wangjordy.github.io,chenbojian/hexo,kywk/hexi,zhipengyan/hexo,registercosmo/hexo,ppker/hexo,Bob1993/hexo,k2byew/hexo,Gtskk/hexo,tzq668766/hexo,DevinLow/DevinLow.github.io,zhi1ong/hexo,imjerrybao/hexo,magicdawn/hexo,xcatliu/hexo,xushuwei202/hexo,zhoulingjun/hexo,wenzhucjy/hexo,HcySunYang/hexo,gaojinhua/hexo,noname007/hexo-1,dieface/hexo,biezhihua/hexo,jollylulu/hexo,wyfyyy818818/hexo,zoubin/hexo,JasonMPE/hexo,wwff/hexo,iamprasad88/hexo,cjwind/hexx,luodengxiong/hexo,wangjordy/wangjordy.github.io,glabcn/hexo,haoyuchen1992/hexo,lukw00/hexo,meaverick/hexo,amobiz/hexo,tibic/hexo,GGuang/hexo,Carbs0126/hexo,chenzaichun/hexo,hexojs/hexo,hexojs/hexo,Jeremy017/hexo,dreamren/hexo,leelynd/tapohuck,leikegeek/hexo,elegantwww/hexo,memezilla/hexo,lknny/hexo,Regis25489/hexo,DanielHit/hexo,liuhongjiang/hexo,sailtoy/hexo,G-g-beringei/hexo,zhipengyan/hexo,kennethlyn/hexo,maominghui/hexo,ppker/hexo,lookii/looki,ChaofengZhou/hexo,karenpeng/hexo,zongkelong/hexo,logonmy/hexo,initiumlab/hexo,noikiy/hexo,HiWong/hexo,SaiNadh001/hexo,kywk/hexi,viethang/hexo,nextexit/hexo,DevinLow/DevinLow.github.io,crystalwm/hexo,r4-keisuke/hexo,Richardphp/hexo,aulphar/hexo,znanl/znanl