fix issue#207: common/utils/net_test.go:25 cannot pass

[beiwei30]

What this PR does:

Which issue(s) this PR fixes:

Fixes #207

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-io]

Codecov Report

Merging #208 into master will increase coverage by 1.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   66.62%   67.66%   +1.04%     
==========================================
  Files          94       94              
  Lines        5821     5824       +3     
==========================================
+ Hits         3878     3941      +63     
+ Misses       1541     1478      -63     
- Partials      402      405       +3

Impacted Files	Coverage Δ
common/utils/net.go	68.57% <66.66%> (-3.31%)	⬇️
registry/etcdv3/registry.go	56.89% <0%> (+1.14%)	⬆️
cluster/cluster_impl/failback_cluster_invoker.go	80.64% <0%> (+2.15%)	⬆️
protocol/dubbo/readwriter.go	70.37% <0%> (+2.46%)	⬆️
protocol/dubbo/codec.go	82.35% <0%> (+5.88%)	⬆️
protocol/dubbo/listener.go	64.02% <0%> (+14.02%)	⬆️
protocol/dubbo/pool.go	73.6% <0%> (+15.73%)	⬆️

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