Fix the bug that Chinese characters are garbled in ResponseBody of ZuulBlockFallbackProvider

[yinjihuan]

Describe what this PR does / why we need it

Fix the bug that Chinese characters are garbled in ResponseBody of ZuulBlockFallbackProvider.

Does this pull request fix one issue?

Fixes #733

Describe how you did it

Add a line ctx.getResponse().setContentType("application/json; charset=utf-8");

Describe how to verify it

Custom ZuulBlockFallbackProvider, Set the message field to Chinese，Trigger current limiting，Message field Chinese display is normal

Special notes for reviews

NO

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #737 into master will increase coverage by 0.06%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #737      +/-   ##
============================================
+ Coverage     40.96%   41.02%   +0.06%     
- Complexity     1335     1337       +2     
============================================
  Files           301      301              
  Lines          8630     8631       +1     
  Branches       1159     1159              
============================================
+ Hits           3535     3541       +6     
+ Misses         4668     4664       -4     
+ Partials        427      426       -1

Impacted Files	Coverage Δ	Complexity Δ
...er/gateway/zuul/filters/SentinelZuulPreFilter.java	14.58% <0%> (-0.32%)	3 <0> (ø)
...tinel/slots/block/flow/param/ParamFlowChecker.java	55.4% <0%> (+2.7%)	29% <0%> (+1%)	⬆️
...m/alibaba/csp/sentinel/log/DateFileLogHandler.java	57.57% <0%> (+3.03%)	7% <0%> (+1%)	⬆️

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[sczyh30]

LGTM

[sczyh30]

Thanks for contributing!