FFT of Complex CircShiftVector Error

These all work:
```
using FFTW, ShiftedArrays

realVec = rand(16)
fft(realVec)
fft(CircShiftedArray(realVec, 16))
fft(view(CircShiftedArray(realVec, 16), 1:4))

cplxVec = rand(ComplexF64, 16)
fft(cplxVec)
```

These do not:
```
fft(CircShiftedArray(cplxVec, 16))
fft(view(CircShiftedArray(cplxVec, 16), 1:4))
```

Naively, the problem seems to be a general method for `plan_fft(x::AbstractArray{T, N}) where T<:Complex` not being defined. A method exists for `fft(x::AbstractArray)` exists, which makes this all a little confusing.

I think #85 and #115 are related to the same core issue.

Does anyone with more of a working knowledge of FFTW understand what the issue is here?