Fix mapexponents! with different variables

Author: blegat

src/cmult.jl

@@ -114,7 +114,7 @@ function MP.mapexponents_to!(output::Monomial{true}, f::Function, x::Monomial{tr
     return output
 end
 function MP.mapexponents!(f::Function, x::Monomial{true}, y::Monomial{true})
-    multdivmono!(x.z, x.vars, x.vars, y, f, x.z)
+    multdivmono!(x.z, x.vars, copy(x.vars), y, f, x.z)
     return x
 end
 function MP.mapexponents(f::Function, x::Monomial{true}, y::Monomial{true})

test/mono.jl

@@ -133,4 +133,7 @@
         @test (x^2)(3) == 9
         @test (x)(3) == 3
     end
+    @testset "TODO remove when added to MP" begin
+        @test x == DynamicPolynomials.MP.mapexponents!(div, x^1, x * y^2)
+    end
 end