Replaced deprecated getheader in favor of headers

[andruten]

May fix #183 deprecation warning. urrlib3 response.headers property is available in all versions supported by django-revproxy.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e7ad0c2) 98.55% compared to head (55e1e9c) 98.55%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #184   +/-   ##
=======================================
  Coverage   98.55%   98.55%           
=======================================
  Files           7        7           
  Lines         414      414           
  Branches       70       70           
=======================================
  Hits          408      408           
  Misses          3        3           
  Partials        3        3           

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