keplerian planets are singular in retrogate 0 inclination cases

[darioizzo]

position=[10000000, 0,0] 
velocity=[0, -8000, 0]
epoch=pk.epoch(0)
prova=pk.planet.keplerian(pk.epoch(0),position,velocity,pk.MU_EARTH,1.0,1.0,1.0,"prova")
prova.eph(pk.epoch(0))```

this returns nans! Note inclination here is 3.14, not zero as checked!