chore: replace APPLICATION_JSON with TEXT_PLAIN

[simon824]

refer to apache/incubator-hugegraph#1888

[codecov]

Codecov Report

Merging #280 (3e837a3) into master (6e39907) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #280   +/-   ##
=========================================
  Coverage     77.89%   77.89%           
  Complexity      972      972           
=========================================
  Files           156      156           
  Lines          4080     4080           
  Branches        242      242           
=========================================
  Hits           3178     3178           
  Misses          723      723           
  Partials        179      179           

Impacted Files	Coverage Δ
...java/com/baidu/hugegraph/api/graphs/GraphsAPI.java	68.08% <100.00%> (ø)

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