fix: [direct_call] Allows the response type to be something else than a JSON (e.g. csv)

[mokaddem]

PyMISP always expects the response to be of JSON type. However, if you specify the returnFormat in MISP to be, for example, csv, PyMISP will throw an error.
If you see a cleaner way to solve that issue, please say so :)

[coveralls]

Coverage decreased (-0.03%) to 51.719% when pulling 98ce3f8 on lenientResponseType into 121d885 on master.

[coveralls]

Coverage decreased (-0.03%) to 51.719% when pulling 98ce3f8 on lenientResponseType into 121d885 on master.

[coveralls]

Coverage decreased (-0.03%) to 51.719% when pulling 98ce3f8 on lenientResponseType into 121d885 on master.

[codecov]

Codecov Report

Merging #395 into master will decrease coverage by 0.03%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
- Coverage   51.75%   51.71%   -0.04%     
==========================================
  Files          35       35              
  Lines        6454     6458       +4     
==========================================
  Hits         3340     3340              
- Misses       3114     3118       +4

Impacted Files	Coverage Δ
pymisp/api.py	42.02% <20%> (-0.06%)	⬇️
pymisp/aping.py	10.83% <33.33%> (-0.06%)	⬇️

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[Rafiot]

Sounds good to me.