fix delegate sometimes not release

[wangliangliang2]

No description provided.

[codecov]

Codecov Report

Merging #264 into master will decrease coverage by 0.05%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
- Coverage   71.43%   71.38%   -0.06%     
==========================================
  Files          31       31              
  Lines        2370     2369       -1     
  Branches      156      156              
==========================================
- Hits         1693     1691       -2     
  Misses        670      670              
- Partials        7        8       +1

Impacted Files	Coverage Δ
QiniuSDK/Http/QNSessionManager.m	86.41% <100%> (-0.15%)	⬇️
QiniuSDKTests/QNFileRecorderTest.m	94.02% <66.66%> (-1.43%)	⬇️

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[longbai]

LGTM