fixes #4039.

Issues is that $(selector).val() return the associated text if no data
is present (hence one cannot associate explicitely an option to
"undefined")

fix in doc and example.
Merge pull request #4253 from Carreau/fix-4039

fixes #4039.

Issues is that $(selector).val() return the associated text if no data
is present (hence one cannot associate explicitely an option to
"undefined")

fix in doc and example.