Fix health check endpoint

[zhouzhuojie]

Fix #261

The reason is that gzip middleware zips empty response without setting content-type, which was empty set in the health swagger definition.

This pr adds a JSON representation of the health check status:

{
  "status": "OK"
}

Which may be extended to other things from flagr server.

[codecov-io]

Codecov Report

Merging #262 into master will decrease coverage by 0.1%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
- Coverage      82%   81.89%   -0.11%     
==========================================
  Files          26       26              
  Lines        1528     1530       +2     
==========================================
  Hits         1253     1253              
- Misses        207      209       +2     
  Partials       68       68

Impacted Files	Coverage Δ
pkg/handler/handler.go	82% <0%> (-3.42%)	⬇️

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[zhouzhuojie]

Plan to migrate to npm only in the near future.

[zhouzhuojie]

Tested locally.

curl -i localhost:18000/api/v1/health
HTTP/1.1 200 OK
Content-Type: application/json
Vary: Origin
Date: Fri, 24 May 2019 21:31:09 GMT
Content-Length: 16

{"status":"OK"}