Allow to overwrite the response Content-Type from options

[BoboTiG]

It implements the support for custom mime types.
It is a start, it will be used later in #1110 to support custom a charset.

Summary:

$ http --response-as 'appllication/xml' \
    https://raw.githubusercontent.com/httpie/httpie/master/tests/fixtures/xmldata/valid/simple_raw.xml

[codecov-commenter]

Codecov Report

Merging #1134 (19cc12b) into master (4d7d6b6) will decrease coverage by 0.21%.
The diff coverage is 94.88%.

@@            Coverage Diff             @@
##           master    #1134      +/-   ##
==========================================
- Coverage   97.28%   97.07%   -0.22%     
==========================================
  Files          67       70       +3     
  Lines        4235     4405     +170     
==========================================
+ Hits         4120     4276     +156     
- Misses        115      129      +14     

Impacted Files	Coverage Δ
tests/test_output.py	99.47% <ø> (ø)
tests/conftest.py	75.75% <50.00%> (-11.20%)	⬇️
tests/test_ssl.py	89.88% <63.63%> (-5.05%)	⬇️
httpie/output/formatters/colors.py	92.66% <83.33%> (-0.92%)	⬇️
httpie/output/utils.py	97.87% <97.87%> (ø)
httpie/__init__.py	100.00% <100.00%> (ø)
httpie/cli/argparser.py	95.60% <100.00%> (-0.76%)	⬇️
httpie/cli/constants.py	100.00% <100.00%> (ø)
httpie/cli/definition.py	100.00% <100.00%> (ø)
httpie/cli/requestitems.py	91.17% <100.00%> (ø)
... and 17 more

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[jkbrzt]

How about --response-as / --treat-response-as?

[BoboTiG]

What would give both options when the charset is set?

--response-content-type='application/xml; charset=utf-8'

--response-as='application/xml; charset=utf-8'

🤔

I kind of like the --response-as. We could go with it if it is OK for you too when there is the charset specified also.

[jkbrzt]

@BoboTiG I think --response-as=<content-type> works for the charset use case as well. Let’s go with it: if we end up not liking it, we can still easily change the name before the next release. Btw, you’ll be adding the charset docs for this option once the #1110 is finished, right?

[BoboTiG]

Yes, charset code & doc have to be done with #1110.