Use of `expint` with ForwardDiff

[mancellin]

While trying to ForwardDiff expint, I got the following error:

  MethodError: no method matching floatmax(::ForwardDiff.Dual{ForwardDiff.Tag{var"#135#136", Float64}, Float64, 3})

occurring at

SpecialFunctions.jl/src/expint.jl

Line 169 in c400278

scale = sqrt(floatmax(real(A)))

The issue is that ForwardDiff.jl only implements floatmax(T), while SpecialFunctions.jl uses floatmax(::T).
In Base, both are implemented, but only floatmax(T) is documented.

There are two possible straightforward solutions in this case (implementing floatmax(::T) in ForwardDiff, or adding a typeof in SpecialFunctions). Not sure if one of them should be preferred.

[ararslan]

I was surprised at first that this didn't hit the Base method for values that just dispatches to calling floatmax on the type, but I realized that the fallback is only for AbstractFloat. SpecialFunctions is surely not the only user of the method for values so it seems like adding such a method to ForwardDiff would be a better solution.