Add supports for ObjectiveFunction (#36)

* Add supports for ObjectiveFunction

* Add test

Author: blegat

src/FlatZinc/model.jl

@@ -102,6 +102,8 @@ function MOI.get(model::Model, ::MOI.ObjectiveFunction{MOI.VariableIndex})
     return model.objective_function
 end
 
+MOI.supports(::Model, ::MOI.ObjectiveFunction{MOI.VariableIndex}) = true
+
 function MOI.set(
     model::Model,
     ::MOI.ObjectiveFunction{MOI.VariableIndex},

test/FlatZinc/model.jl

@@ -7,6 +7,10 @@
     @testset "Supported constraints" begin
         m = CP.FlatZinc.Model()
 
+        @test MOI.supports(
+            m,
+	    MOI.ObjectiveFunction{MOI.VariableIndex}(),
+        )
         @test MOI.supports_constraint(
             m,
             MOI.VariableIndex,
@@ -105,4 +109,4 @@
         @test MOI.supports_add_constrained_variable(m, S)
         @test MOI.supports_add_constrained_variables(m, S)
     end
-end
+end