Fix Expression::fix_variables() for quadratic expressions

[arcondello]

Closes https://github.com/dwavesystems/dimod/issues/1351

[codecov[bot]]

Codecov Report

Merging #1352 (4128012) into main (760b693) will increase coverage by 0.10%. The diff coverage is 98.30%.

@@            Coverage Diff             @@
##             main    #1352      +/-   ##
==========================================
+ Coverage   94.97%   95.08%   +0.10%     
==========================================
  Files          95       95              
  Lines        9852     9902      +50     
==========================================
+ Hits         9357     9415      +58     
+ Misses        495      487       -8     

Impacted Files	Coverage Δ
dimod/include/dimod/constrained_quadratic_model.h	94.11% <92.30%> (+3.20%)	:arrow_up:
dimod/include/dimod/expression.h	87.61% <100.00%> (+0.17%)	:arrow_up:
...estscpp/tests/test_constrained_quadratic_model.cpp	100.00% <100.00%> (ø)

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