fix(ASGI): assume ISO-8859-1 for ASGI headers

[vytas7]

Closes #1911

[codecov]

Codecov Report

Merging #1914 (7ceb7a5) into master (e1ebdb7) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1914   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           63        63           
  Lines         6593      6610   +17     
  Branches      1065      1067    +2     
=========================================
+ Hits          6593      6610   +17     

Impacted Files	Coverage Δ
falcon/asgi/_request_helpers.py	100.00% <100.00%> (ø)
falcon/asgi/request.py	100.00% <100.00%> (ø)
falcon/asgi/response.py	100.00% <100.00%> (ø)
falcon/testing/helpers.py	100.00% <100.00%> (ø)
falcon/util/misc.py	100.00% <100.00%> (ø)

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[vytas7]

I realized I probably need to add a couple more regression tests for the reported issue itself. Working on this now.

Edit: done now.