Feature / Fix: Use charset provided by web server instead of utf-8

[DerDreschner]

When downloading a calendar file from a web server, it can happen the charset isn't utf-8 (like in #104). We should use that charset for decoding the downloaded content instead of using UTF-8.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.02%. Comparing base (4d29ced) to head (abebd44).
Report is 28 commits behind head on master.

Files with missing lines	Patch %	Lines
icalevents/icaldownload.py	83.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   91.82%   91.02%   -0.80%     
==========================================
  Files           4        4              
  Lines         379      379              
  Branches      100      101       +1     
==========================================
- Hits          348      345       -3     
- Misses         13       15       +2     
- Partials       18       19       +1     

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