Fix failed MOI tests

[metab0t]

I applied the same tricks used in Xpress.jl here to fix 3 failed MOI tests.
The test_solve_conflict_feasible fails because Gurobi cannot compute IIS on a feasible model

[odow]

Can you fix the format-check please?

[metab0t]

I think objective_type should be a nullable type like objective_sense.

is_objective_set can be discarded and objective_type = nothing represents no objective function is set.

The complicated thing is here. We need to check the corresponding variable of changed coefficient and set objective_type if objective_type = _SINGLE_VARIABLE currently.

function MOI.modify(
    model::Optimizer,
    ::MOI.ObjectiveFunction{MOI.ScalarAffineFunction{Float64}},
    chg::MOI.ScalarCoefficientChange{Float64},
)
    ret = GRBsetdblattrelement(
        model,
        "Obj",
        Cint(column(model, chg.variable) - 1),
        chg.new_coefficient,
    )
    _check_ret(model, ret)
    model.is_objective_set = true
    _require_update(model)
    return
end

[odow]

I think this is okay for now.