add content-type response headers

[ubogdan]

Describe the PR
add content-type response header

Relation issue
NONE

Additional context
none

[codecov]

Codecov Report

Merging #55 (bd37c4b) into master (4b2426b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #55   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           63        73   +10     
=========================================
+ Hits            63        73   +10     

Impacted Files	Coverage Δ
swagger.go	100.00% <100.00%> (ø)

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