Preserve individual headers with the same name on responses

[isidentical]

Resolves #1207

[codecov-commenter]

Codecov Report

Merging #1208 (47311d1) into master (4d7d6b6) will decrease coverage by 0.49%.
The diff coverage is 94.32%.

❗ Current head 47311d1 differs from pull request most recent head b0c55b0. Consider uploading reports for the commit b0c55b0 to get more accurate results

@@            Coverage Diff             @@
##           master    #1208      +/-   ##
==========================================
- Coverage   97.28%   96.78%   -0.50%     
==========================================
  Files          67       72       +5     
  Lines        4235     4637     +402     
==========================================
+ Hits         4120     4488     +368     
- Misses        115      149      +34     

Impacted Files	Coverage Δ
tests/test_auth.py	100.00% <ø> (ø)
tests/test_binary.py	100.00% <ø> (ø)
httpie/compat.py	16.66% <9.09%> (-83.34%)	⬇️
tests/conftest.py	76.47% <54.54%> (-10.49%)	⬇️
tests/test_ssl.py	91.01% <63.63%> (-3.93%)	⬇️
httpie/models.py	95.31% <75.00%> (-2.06%)	⬇️
httpie/output/formatters/colors.py	92.66% <83.33%> (-0.92%)	⬇️
tests/utils/http_server.py	97.14% <97.14%> (ø)
httpie/__init__.py	100.00% <100.00%> (ø)
httpie/adapters.py	100.00% <100.00%> (ø)
... and 39 more

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[isidentical]

I am thinking about moving httpie/ssl.py into httpie/adapters.py, since it does make sense for all the adapters to be in the same place. WDYT @jakubroztocil?

[jkbrzt]

Sounds good 👍🏻