Fix bug happen when config/wechat.yml have no default session.
Fix bug happen when config/wechat.yml have no default session.

Eric-Guo/wechat,yxf/wechat,Dearest/wechat-1,skinnyworm/wechat-rails,zfben/wechat,wolflee/wechat-rails,EndingChaung/wechat,JeeLiu/wechat,wolflee/wechat-rails,Dearest/wechat-1,Eric-Guo/wechat,TangMonk/wechat,Dearest/wechat-1