Safely initialize the inplace Jacobian cache

This fixes support for BigFloat in LsqFit.jl. Making a copy instead of similar ensures that the elements aren't `undef`. This is what previous versions did.

Author: JamesWrigley

src/objective_types/oncedifferentiable.jl

@@ -78,7 +78,7 @@ function OnceDifferentiable(f, x_seed::AbstractArray, F::AbstractArray, DF::Abst
         fdfF = make_fdf(f, x_seed, F)
         return OnceDifferentiable(fF, dfF, fdfF, x_seed, F, DF)
     else
-        F2 = similar(F)
+        F2 = copy(F)
         backend = get_adtype(autodiff, chunk)
         jac_prep = DI.prepare_jacobian(f, F2, backend, x_seed)
         function j!(_j, _x)