Implement support for multiple headers with the same name in sessions

[isidentical]

Fixes #1331

[codecov-commenter]

Codecov Report

Merging #1335 (6d8ee7f) into master (4d7d6b6) will decrease coverage by 0.45%.
The diff coverage is 96.14%.

@@            Coverage Diff             @@
##           master    #1335      +/-   ##
==========================================
- Coverage   97.28%   96.83%   -0.46%     
==========================================
  Files          67       96      +29     
  Lines        4235     6821    +2586     
==========================================
+ Hits         4120     6605    +2485     
- Misses        115      216     +101     

Impacted Files	Coverage Δ
tests/test_binary.py	100.00% <ø> (ø)
httpie/compat.py	31.11% <27.90%> (-68.89%)	⬇️
tests/conftest.py	77.14% <61.53%> (-9.82%)	⬇️
httpie/manager/__main__.py	82.35% <82.35%> (ø)
httpie/output/lexers/metadata.py	82.35% <82.35%> (ø)
httpie/output/lexers/http.py	86.36% <86.36%> (ø)
tests/test_ssl.py	92.66% <87.50%> (-2.28%)	⬇️
httpie/models.py	95.19% <90.00%> (-2.18%)	⬇️
httpie/manager/tasks/sessions.py	92.45% <92.45%> (ø)
tests/test_uploads.py	97.70% <92.59%> (-2.30%)	⬇️
... and 81 more

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