fix convert(Variable, D(x)) (#412)

Fix #409

Author: YingboMa

src/ModelingToolkit.jl

@@ -73,7 +73,7 @@ function Base.convert(::Type{Variable},x::Operation)
         x.op
     elseif x.op isa Differential
         var = x.args[1].op
-        rename(var,Symbol(var.name,:ˍ,x.args[1].args[1].op.name))
+        rename(var,Symbol(var.name,:ˍ,x.op.x))
     else
         throw(error("This Operation is not a Variable"))
     end

test/derivatives.jl

@@ -86,3 +86,8 @@ tmp = beta * (alpha * exp(x1) * x2 ^ (alpha - 1) + 1 - delta) / x3
 # derivative w.r.t. x1 and x2
 t1 = ModelingToolkit.gradient(tmp, [x1, x2])
 @test_nowarn ModelingToolkit.gradient(t1[1], [beta])
+
+@parameters t k
+@variables x(t)
+@derivatives D'~k
+@test convert(Variable, D(x)).name === :xˍk