Fix dual objective value with free variable

[odow]

Closes #254

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.78%. Comparing base (6914b62) to head (9d3c9c0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
+ Coverage   99.71%   99.78%   +0.07%     
==========================================
  Files           1        1              
  Lines        1397     1398       +1     
==========================================
+ Hits         1393     1395       +2     
+ Misses          4        3       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.