gzip-compressed mimetype if download one compressed file

[dsavchenko]

To fix wrong mimetype mentioned in #724 (comment)

Also includes marshmellow version restriction (new incompatible update is several days old). Break is due to use of "private" variable here

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.09%. Comparing base (a45dccd) to head (8f44325).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #726   +/-   ##
=======================================
  Coverage   62.09%   62.09%           
=======================================
  Files          50       50           
  Lines        9167     9168    +1     
=======================================
+ Hits         5692     5693    +1     
  Misses       3475     3475           

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[burnout87]

anything to comment on this @volodymyrss ?

[dsavchenko]

@volodymyrss this has priority: the main change itself is minor, but the marshmallow version restriction is important.
Please review

[volodymyrss]

thanks for reminding!