Fix Allocate-Load copy for single variable constraints

[blegat]

The change in MockOptimizer is to makes
https://github.com/JuliaOpt/MathOptInterface.jl/blob/6ac4e6d6ee47486062ab9a14816aca6d51328760/test/Utilities/copy.jl#L51-L58
fail and the change in copy.jl fixes it.
This fixes the bug reported by @lkapelevich in https://gist.github.com/lkapelevich/9942e37d9eb3cef96d6be7eb041ee838

[codecov-io]

Codecov Report

Merging #856 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #856      +/-   ##
==========================================
+ Coverage   94.95%   94.99%   +0.03%     
==========================================
  Files          72       72              
  Lines        7942     8024      +82     
==========================================
+ Hits         7541     7622      +81     
- Misses        401      402       +1

Impacted Files	Coverage Δ
src/Utilities/mockoptimizer.jl	89.89% <ø> (ø)	⬆️
src/Utilities/copy.jl	94.44% <100%> (+1.72%)	⬆️

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