Fix bug when copying model with named expressions

[odow]

Closes #2403

[codecov]

Codecov Report

Merging #2405 (5f991d8) into master (90e63c4) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2405      +/-   ##
==========================================
+ Coverage   91.67%   91.86%   +0.19%     
==========================================
  Files          42       42              
  Lines        4456     4462       +6     
==========================================
+ Hits         4085     4099      +14     
+ Misses        371      363       -8     

Impacted Files	Coverage Δ
src/copy.jl	88.23% <100.00%> (+3.61%)	⬆️
src/quad_expr.jl	96.20% <0.00%> (+1.23%)	⬆️
src/aff_expr.jl	95.20% <0.00%> (+4.04%)	⬆️

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