server_response: added Content-Type for serialized json

[codecov-commenter]

Codecov Report

Merging #55 (3206616) into main (71febea) will decrease coverage by 0.18%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   80.99%   80.81%   -0.18%     
==========================================
  Files           7        7              
  Lines         563      563              
==========================================
- Hits          456      455       -1     
- Misses        107      108       +1     

Files Changed	Coverage Δ
src/server.jl	77.41% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[rssdev10]

This fix allows to properly see a serialized to JSON result. Otherwise, the Postman and browser dev tools may show it as plain text.

[tanmaykm]

Thanks!