Refine abstract methods and properties

[BoboTiG]

Extracted from #1110.

[codecov-commenter]

Codecov Report

Merging #1118 (e3982f1) into master (de13423) will increase coverage by 0.46%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1118      +/-   ##
==========================================
+ Coverage   96.48%   96.95%   +0.46%     
==========================================
  Files          64       64              
  Lines        4128     4172      +44     
==========================================
+ Hits         3983     4045      +62     
+ Misses        145      127      -18     

Impacted Files	Coverage Δ
httpie/models.py	97.33% <100.00%> (+5.44%)	⬆️
httpie/output/streams.py	100.00% <100.00%> (+9.70%)	⬆️
tests/test_stream.py	100.00% <0.00%> (ø)
tests/test_unicode.py	100.00% <0.00%> (ø)
httpie/plugins/manager.py	91.89% <0.00%> (+2.70%)	⬆️
httpie/plugins/base.py	88.57% <0.00%> (+2.85%)	⬆️
httpie/output/processing.py	100.00% <0.00%> (+6.06%)	⬆️

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