feat(static): set `Content-Length` for static file responses

[vytas7]

Also fix range request support across different WSGI servers by always providing a Python wrapper without a file descriptor, since many sendfile implementations are buggy wrt not respecting the current file offset; both Gunicorn (https://github.com/benoitc/gunicorn/issues/2669) and uWSGI are affected according to my testing.

There is still an open question remaining what to do with Meinheld: I've filed this as a separate issue (#1992).

[codecov[bot]]

Codecov Report

Merging #1991 (ca4e279) into master (73b90c2) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1991   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           63        63           
  Lines         6669      6674    +5     
  Branches      1079      1079           
=========================================
+ Hits          6669      6674    +5     

Impacted Files	Coverage Δ
falcon/app.py	100.00% <ø> (ø)
falcon/routing/static.py	100.00% <100.00%> (ø)

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[vytas7]

We already have unit tests covering ASGI too, but I will check if all the functionality is verified in unit tests as well.

Otherwise, ASGI has no concept of sendfile, and the response is rendered purely by the app, so adequate unit testing should be enough.