Correctly handle single-byte Content-Range

[blyxxyz]

HTTPie fails if it continues a download with a single byte left. For example:

$ http -b -d example.org
Downloading 1.23 kB to "index.html"
Done. 1.23 kB in 0.00043s (2.81 MB/s)
$ python3 -c 'import sys; sys.stdout.write(open("index.html").read()[:-1])' > index-short.html
$ http -b -d -c -o index-short.html example.org

http: error: ContentRangeError: Invalid Content-Range returned: 'bytes 1255-1255/1256'

The range is inclusive on both ends.

[codecov-io]

Codecov Report

Merging #1032 (fc7c396) into master (e944dbd) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1032   +/-   ##
=======================================
  Coverage   95.58%   95.58%           
=======================================
  Files          61       61           
  Lines        3967     3967           
=======================================
  Hits         3792     3792           
  Misses        175      175           

Impacted Files	Coverage Δ
httpie/downloads.py	91.22% <100.00%> (ø)
tests/test_downloads.py	100.00% <100.00%> (ø)

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[jkbrzt]

@blyxxyz good catch. Thanks for the PR and test! 🥧