FileFormats.MPS: add option to not write OBJSENSE

[odow]

Introduced in #1391, but broke joaquimg/BilevelJuMP.jl#170.

We should add an option to skip

MathOptInterface.jl/src/FileFormats/MPS/MPS.jl

Lines 174 to 178 in 6b50fa0

	if MOI.get(model, MOI.ObjectiveSense()) == MOI.MAX_SENSE
	println(io, "OBJSENSE MAX")
	else
	println(io, "OBJSENSE MIN")
	end

and negate the objective coefficients.

cc @joaquimg

[joaquimg]

If we want to be strict on writing, the default should be to skip that code.