Proper JSON handling for :=/:=@

[isidentical]

Fixes #1212

[codecov-commenter]

Codecov Report

Merging #1213 (664d21e) into master (4d7d6b6) will decrease coverage by 0.53%.
The diff coverage is 94.25%.

@@            Coverage Diff             @@
##           master    #1213      +/-   ##
==========================================
- Coverage   97.28%   96.75%   -0.54%     
==========================================
  Files          67       72       +5     
  Lines        4235     4677     +442     
==========================================
+ Hits         4120     4525     +405     
- Misses        115      152      +37     

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	94.73% <83.33%> (-2.64%)	⬇️
httpie/output/formatters/colors.py	92.66% <83.33%> (-0.92%)	⬇️
httpie/cli/requestitems.py	90.69% <92.00%> (-0.48%)	⬇️
tests/utils/http_server.py	97.14% <97.14%> (ø)
httpie/__init__.py	100.00% <100.00%> (ø)
... and 39 more

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[isidentical]

@jakubroztocil do you have any idea about why the links from the coverage bot is not working? I can't see what part decreased the cover, since every link I click is broken.

[jkbrzt]

Looks good 👍🏻 Tweaked the docs a bit. Please also add a CHANGELOG entry.

[jkbrzt]

Suggested change

	Be aware that when using this syntax with form data (`--form` is activated);
	only strings, numbers and floats are enabled. Rest of the JSON types are not
	available.
	The `:=` syntax is JSON-specific. You can switch your request to `--form` or `--multipart`,
	and string, float, and number values will continue to be serialized (as string form values).
	Other JSON types, however, are not allowed with `--form` or `--multipart`.