`NaNMath.sqrt` has no method for complex arguments

[OsmarCLFilho]

Given a + bim, we could just return Base.sqrt(a + bim) since it can handle those values without throwing domain errors.

Currently, NaNMath.sqrt(a + bim) will just yield a missing method error.

[moble]

This is essentially the same as #66.

[oameye]

resolved in #71