handlers: Add "X-Attribute-*" header for OPTIONS upload

[tatiana-nspcc]

Closes #187.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 22.95%. Comparing base (067e507) to head (74980dc).
Report is 1 commits behind head on master.

Files	Patch %	Lines
handlers/preflight.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #188   +/-   ##
=======================================
  Coverage   22.95%   22.95%           
=======================================
  Files          14       14           
  Lines        3572     3572           
=======================================
  Hits          820      820           
  Misses       2631     2631           
  Partials      121      121           

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[roman-khimov]

How it was tested?

[tatiana-nspcc]

How it was tested?

I've locally run to check that it returns "X-Bearer-For-All-Users, X-Bearer-Lifetime, X-Bearer-Owner-Id, X-Bearer-Signature, X-Bearer-Signature-Key, Content-Type, Authorization, X-Attribute-*" in Access-Control-Allow-Headers. That's all for a while.