BUG: buttap() should produce a purely real pole
BUG: buttap() should produce a purely real pole

Previously buttap for odd orders produced a pole like -1+6.1e-17j rather than -1+0j exactly.  This made poly(p) complex, though all the imaginary parts were negligible.  This then made the ba representation complex, though it is a purely real filter.

Now the calculation is symmetrical, so the middle term works out to -exp(0j), which is exactly -1+0j.

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