Fix lowess for constant inputs.

Fix with tests for #1798.
This also sets the non-zero resid to zero. At least half of the resid are zero, but the other observations are "outliers"

I think it should be `std_resid[std_resid > 0] = 1.0`  so that the weight on the observations with nonzero resid becomes 0 in bisquare.

Needs a test case with  some non-zero observations.