handlers: No body response for OPTIONS requests

[tatiana-nspcc]

Close #166.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 22.94%. Comparing base (57d165d) to head (a5d0acf).
Report is 16 commits behind head on master.

Files	Patch %	Lines
handlers/preflight.go	0.00%	12 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #169   +/-   ##
=======================================
  Coverage   22.94%   22.94%           
=======================================
  Files          14       14           
  Lines        3574     3574           
=======================================
  Hits          820      820           
  Misses       2633     2633           
  Partials      121      121           

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