Explicitly add content type header.

[ninzavivek]

If CNS Client happen to use a library which checks for the content type before decoding the response ( ex: Python -aiohttp, then reading the response will always fail.

We are defaulting to "Content-Type: text/plain; charset=utf-8"

As the response is always JSON ended. The fix basically always add the Content-Type header as "application/json; charset=UTF-8"

[codecov]

Codecov Report

Merging #561 into master will decrease coverage by 1.37%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #561      +/-   ##
==========================================
- Coverage   49.22%   47.85%   -1.38%     
==========================================
  Files          28       27       -1     
  Lines        3437     3281     -156     
==========================================
- Hits         1692     1570     -122     
+ Misses       1455     1434      -21     
+ Partials      290      277      -13     

[chandanAggarwal]

[ninzavivek]

Considering the response is always JSON encoded, I think it is fine.