I am using Measurements.jl for error propagation in my code and have come across an error when trying to use it with a Bessel function with a complex input.

This works

```
using Measurements
x = 1 ± 0.5
y = besselk(1. , x)
```

and this works

```
y = besselk(1., complex(1.,1.))
```

But this does not

```
using Measurements
x = 1 ± 0.5
y = besselk(1. , complex(1. , x))
```

and gives the following error

LoadError: MethodError: no method matching besselk(::Measurements.Measurement{Float64}, ::Complex{Measurements.Measurement{Float64}})

Closest candidates are:

besselk(::T<:AbstractFloat, ::Complex{T<:AbstractFloat}) where T<:AbstractFloat at C:\Users\xxxxxx.julia\v0.6\SpecialFunctions\src\bessel.jl:457

besselk(::Real, ::Complex) at C:\Users\xxxxxx.julia\v0.6\SpecialFunctions\src\bessel.jl:454

besselk(::Any…; kwargs…) at deprecated.jl:1294

…

while loading TestCase.jl, in expression starting on line 7

in besselk at SpecialFunctions\src\bessel.jl:455

I was under the impression that Measurements.jl was compatible with Julia functions that used AbstractFloat data types?

Is there something I am missing?