Fix printing of non-real number types

[odow]

Closes #1693

julia> model = MOI.Utilities.UniversalFallback(MOI.Utilities.Model{Float64}())
MOIU.UniversalFallback{MOIU.Model{Float64}}
fallback for MOIU.Model{Float64}

julia> x = MOI.add_variable(model)
MathOptInterface.VariableIndex(1)

julia> c = MOI.add_constraint(model, (1.0 + 1.0im)x, MOI.EqualTo(1.0 + 1.0im))
MathOptInterface.ConstraintIndex{MathOptInterface.ScalarAffineFunction{ComplexF64}, MathOptInterface.EqualTo{ComplexF64}}(1)

julia> print(model)
Feasibility

Subject to:

ScalarAffineFunction{ComplexF64}-in-EqualTo{ComplexF64}
 (0.0 + 0.0im) + (1.0 + 1.0im) v[1] == (1.0 + 1.0im)