bug fix and add demo

[timhbw]

• resolved 为什么不把某些接口加进来？ #301 ：examples/rs_asynch_fetch.php
• resolved 没有内容审核的sdk吗？ #312 ：examples/censor_image.php、examples/censor_video.php
• fix Response的isJson不支持http2的返回头 #331
• fix：add forceSaveKey in policyFields
  .........

[codecov]

Codecov Report

Merging #341 into master will increase coverage by 25.10%.
The diff coverage is 64.10%.

@@             Coverage Diff             @@
##           master     #341       +/-   ##
===========================================
+ Coverage   60.09%   85.19%   +25.10%     
===========================================
  Files          20       20               
  Lines        1268     1304       +36     
===========================================
+ Hits          762     1111      +349     
+ Misses        506      193      -313     

Impacted Files	Coverage Δ
src/Qiniu/Auth.php	85.18% <ø> (+24.69%)	⬆️
src/Qiniu/Config.php	62.50% <ø> (+7.81%)	⬆️
tests/bootstrap.php	100.00% <ø> (ø)
src/Qiniu/Storage/BucketManager.php	77.41% <63.15%> (+42.49%)	⬆️
src/Qiniu/Http/Response.php	74.50% <100.00%> (ø)
src/Qiniu/Http/Client.php	100.00% <0.00%> (+6.38%)	⬆️
src/Qiniu/Http/Error.php	100.00% <0.00%> (+22.22%)	⬆️
... and 3 more

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[longbai]

LGTM