Set Content-Type in healthcheck's http response

[logeable]

Signed-off-by: logeable logeable@gmail.com

Which problem is this PR solving?

• Resolve The response ContentType of HealthCheck.Handler is not application/json #2924

Short description of the changes

• set header before call WriteHeader

[codecov]

Codecov Report

Merging #2926 (efe7ddb) into master (33e8e96) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2926      +/-   ##
==========================================
+ Coverage   95.91%   95.94%   +0.03%     
==========================================
  Files         223      223              
  Lines        9708     9708              
==========================================
+ Hits         9311     9314       +3     
+ Misses        327      326       -1     
+ Partials       70       68       -2     

Impacted Files	Coverage Δ
pkg/healthcheck/handler.go	100.00% <100.00%> (ø)
...lugin/sampling/strategystore/adaptive/processor.go	99.07% <0.00%> (-0.93%)	⬇️
cmd/query/app/server.go	97.08% <0.00%> (+1.45%)	⬆️
cmd/query/app/static_handler.go	96.77% <0.00%> (+1.61%)	⬆️
cmd/collector/app/server/zipkin.go	76.92% <0.00%> (+3.84%)	⬆️

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[yurishkuro]

Thanks!