Attributes should define is_set_by_optimize

[odow]

See the discussions here:
jump-dev/JuMP.jl#2587
jump-dev/JuMP.jl#2784

[metab0t]

Does jump-dev/JuMP.jl#2587 (comment) solve the problem?
Based on fix of JuMP side above, should we define the following code here to solve this problem?

MOI.is_set_by_optimize(::ModelAttribute) = true
MOI.is_set_by_optimize(::VariableAttribute) = true
MOI.is_set_by_optimize(::ConstraintAttribute) = true

[odow]

Yes, I think that's sufficient for now.