Fix sqrt for complex numbers when x.im == 0

[vilchy]

Currently, if the imaginary part of a complex number is zero, then the imaginary part of the result of sqrt is always zero (because sign(0) = 0):

import complex
sqrt Complex(-1., 0.)
> Complex(0., 0.)

After the fix, sqrt returns the correct values:

import complex
sqrt Complex(-1., 0.)
> Complex(0., 1.)