Fix cases where comparison first and property first queries did not match

[JPBergsma]

Rerouted 'constant_first_comparison' via 'property_first_comparison' in optimade/filtertransformers/mongo.py so $ne and $size are handled properly for constant_first_comparisons.

Closes #1133

[codecov]

Codecov Report

Merging #1134 (464f0b9) into master (0fb4423) will increase coverage by 0.03%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master    #1134      +/-   ##
==========================================
+ Coverage   92.33%   92.37%   +0.03%     
==========================================
  Files          68       68              
  Lines        3849     3881      +32     
==========================================
+ Hits         3554     3585      +31     
- Misses        295      296       +1     

Flag	Coverage Δ
project	92.37% <94.44%> (+0.03%)	⬆️
validator	92.37% <94.44%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
optimade/server/warnings.py	88.88% <50.00%> (+0.65%)	⬆️
optimade/server/query_params.py	98.03% <96.66%> (-1.97%)	⬇️
optimade/filtertransformers/mongo.py	96.66% <100.00%> (ø)
optimade/server/config.py	91.39% <100.00%> (+0.09%)	⬆️
optimade/server/routers/utils.py	98.13% <100.00%> (+0.03%)	⬆️

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[ml-evs]

Could you add a test that fails without these proposed changes, and passes with them? I imagine you have implicitly done this by adding the extra reference test case, but it would be good to see it clearly.

[JPBergsma]

I have added a few test for the MongoDB filter transformer.
The extra reference test case will indeed also trigger this bug.

[ml-evs]

LGTM, thanks