Fix lowess for constant inputs.

Fix with tests for #1798.
Replacing the second `yield` to use `out='out_0'` gives
```
..F
======================================================================
FAIL: test_iter_0
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.4/site-packages/nose/case.py", line 198, in runTest
    self.test(*self.arg)
  File "/usr/lib/python3.4/site-packages/numpy/testing/utils.py", line 454, in assert_almost_equal
    return assert_array_almost_equal(actual, desired, decimal, err_msg)
  File "/usr/lib/python3.4/site-packages/numpy/testing/utils.py", line 811, in assert_array_almost_equal
    header=('Arrays are not almost equal to %d decimals' % decimal))
  File "/usr/lib/python3.4/site-packages/numpy/testing/utils.py", line 644, in assert_array_compare
    raise AssertionError(msg)
AssertionError: 
Arrays are not almost equal to 7 decimals

(mismatch 50.0%)
 x: array([[  0.        ,   1.109194  ],
       [  1.        ,   1.96623384],
       [  2.        ,   2.8223437 ],...
 y: array([[  0.        ,   0.62644795],
       [  1.        ,   1.50083964],
       [  2.        ,   2.38617619],...
```