FIX bug with set_params on minkowski 'p'.

Fixes #2609.

There was a bug that set_params didn't effectively change the value
of minkowski 'p'. Because 'p' was put in self.metric_kwds in
_init_params and then it never changed there.
TST fix doctest for k-NN
DOC deprecation of **kwargs in neighbors per 0.18
Update gaussian_process.py

just being specific as target value instead of value
DOC: Added np.sqrt since default is 'squared=False'
Repeated word: 'the the' -> 'the'

Removed one "the".
FIX: Fix the imputation example
DOC: more explicit title

"Scaling strategies" was ambiguous: it could sound like how to rescale
data.
ENH: minor speed-up in k-means