Use the method for jacobian_sparsity for isautonomous

[ChrisRackauckas]

As @shashi mentions in #810, jacobian_sparsity could be used to speed up isautonomous. While jacobian_sparsity(sys) is wrong, using the call to the jacobian_sparsity calculation itself to get the sparsity w.r.t. t would be what we want (setting all derivatives to zero and checking if the sparsity is trivial).