test that bias has no grads with Zeros

test/layers/conv.jl

@@ -31,6 +31,8 @@ end
   bias = Conv((2,2), 1=>3, bias = Flux.Zeros())
   op = bias(ip)
   @test sum(op) === 0.f0
+  gs = gradient(() -> sum(bias(ip)), Flux.params(bias))
+  @test gs[bias.bias] == nothing
 
   # Train w/o bias and make sure no convergence happens
   # when only bias can be converged