Fix duplicate keys preservation of JSON data

[BoboTiG]

Supersedes #1136 (I messed up with remote repositories).

Fixes #1133.

[codecov-commenter]

Codecov Report

Merging #1163 (21bb027) into master (4d7d6b6) will decrease coverage by 0.01%.
The diff coverage is 99.15%.

@@            Coverage Diff             @@
##           master    #1163      +/-   ##
==========================================
- Coverage   97.28%   97.26%   -0.02%     
==========================================
  Files          67       70       +3     
  Lines        4235     4316      +81     
==========================================
+ Hits         4120     4198      +78     
- Misses        115      118       +3     

Impacted Files	Coverage Δ
httpie/output/formatters/colors.py	92.66% <83.33%> (-0.92%)	⬇️
httpie/__init__.py	100.00% <100.00%> (ø)
httpie/cli/requestitems.py	91.17% <100.00%> (ø)
httpie/output/formatters/json.py	100.00% <100.00%> (ø)
httpie/output/lexers/http.py	100.00% <100.00%> (ø)
httpie/output/lexers/json.py	100.00% <100.00%> (ø)
httpie/output/utils.py	100.00% <100.00%> (ø)
httpie/utils.py	100.00% <100.00%> (ø)
tests/fixtures/__init__.py	100.00% <100.00%> (ø)
tests/test_cli.py	100.00% <100.00%> (ø)
... and 6 more

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