Fix return types in model.jl

[odow]

Some of these were causing unexpected returns from function calls.

Here was an example:

julia> c = MOI.add_constraint(model, MOI.VectorOfVariables(x), MOI.Nonnegatives(2))
MathOptInterface.ConstraintIndex{MathOptInterface.VectorOfVariables,MathOptInterface.Nonnegatives}(2)

julia> MOI.delete(model, c)
Dict{MathOptInterface.ConstraintIndex,String} with 0 entries  # <-- What is this?

[blegat]

I'd rather merge #1245 first or it will create a lot of conflicts

[odow]

Is #1245 ready to review?

I needed this for #1281

[blegat]

Is #1245 ready to review?

Yes :)

[odow]

Closing because this was implemented in #1245