FFTA.plan_fft returns an FFTW plan if both FFTW and FFTA are used

If I run

```julia
using FFTA
a = rand(ComplexF64, 128)
p = FFTA.plan_fft(a)
typeof(p)
```

I get

```julia
FFTA.FFTAPlan_cx{ComplexF64, 1}
```

but if I run

```julia
using FFTA, FFTW
a = rand(ComplexF64, 128)
p = FFTA.plan_fft(a)
typeof(p)
```

I get

```julia
FFTW.cFFTWPlan{ComplexF64, -1, false, 1, Tuple{Int64}}
```

This causes subtle bugs in my code, and presumably is not what is intended? Is this a misunderstanding on my part (on how Julia works) or can this be fixed somehow (in either FFTA, FFTW or AbstractFFTs)?