Prepare release 2.8.6

--HG--
branch : 2.8
Added tag 2.8.6 for changeset 421df39be1c7

--HG--
branch : 2.8
Increase version number

--HG--
branch : 2.8
Fix set_value of Calendar widget

When saving record with focus on calendar widget (which was changed but without
final proper value), the calendar set it to empty instead of trying to complete
the date.

issue3779
review4551002
(grafted from 850084d29b4cd9a3764193861466e9ca50a24922)

--HG--
branch : 2.8
extra : source : 850084d29b4cd9a3764193861466e9ca50a24922
Fix set_client of datetime fields to not fail with None value

Since changeset 850084d29b4c, the widget could use None as value to set.

issue3789
(grafted from 87ea0599ee918e1b8eca01ed43dc3803c8a1f85b)

--HG--
branch : 2.8
extra : source : 87ea0599ee918e1b8eca01ed43dc3803c8a1f85b
Fix set_client of date and time field to convert datetime

issue3795
review4651002
(grafted from 089e0498e0ead7ad03ac383930d28b159daa55f3)

--HG--
branch : 2.8
extra : source : 089e0498e0ead7ad03ac383930d28b159daa55f3