[GoReport Issue] fix issue for Pouch reported from GoReport

[allencloud]

Ⅰ. Issue Description

As an open source project, Pouch is emphasizing much on project quality including code and test and documents.

In code part, we use tool go report to check the quality, which is concluded on https://goreportcard.com/report/github.com/alibaba/pouch.

Here is the entrance which could be clicked:

There are still some work we need to do for Pouch, although Pouch has got a score of A+:

Here, we sincerely invite warm-hearted contributors to help solve this issues. It is really important to improve pouch on all these part. 🌹 🦁 🍺 🌞