replace explicit type with <>

[linyy1991]

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fix #445

[coveralls]

Coverage decreased (-0.01%) to 86.723% when pulling 4782655 on linyy1991:yy into f206a8b on ServiceComb:master.

[WillemJiang]

Merged the patch into master branch by resolving the conflicting files by hand with thanks to linyy1991.