Fix:issue #68

[fangyincheng]

What this PR does:

Which issue(s) this PR fixes:

Fixes #68

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-io]

Codecov Report

Merging #82 into master will increase coverage by 0.52%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage    67.8%   68.33%   +0.52%     
==========================================
  Files          49       49              
  Lines        3414     3414              
==========================================
+ Hits         2315     2333      +18     
+ Misses        868      848      -20     
- Partials      231      233       +2

Impacted Files	Coverage Δ
registry/zookeeper/zk_client.go	54.96% <ø> (ø)	⬆️
registry/zookeeper/registry.go	64.06% <ø> (ø)	⬆️
protocol/dubbo/listener.go	59.06% <100%> (ø)	⬆️
protocol/dubbo/codec.go	77.77% <100%> (ø)	⬆️
protocol/dubbo/pool.go	69.72% <0%> (+9.72%)	⬆️

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[AlexStocks]

LGTM