[Dropdown] - Add "set exactly" behavior

[fpayer]

Expected
The Dropdown should remove existing selected values in the dropdown and replace them with the new ones.

Result
The Dropdown does not set the selected options and appends the newly selected values to the current selection.

Testcase
https://jsfiddle.net/gaa545jc/8/

Notes
Calling "clear" prior to calling "set selected" works as a temporary solution. However, there seems to be some weird effects on "get value" after calling "clear" and "set selected"

[jlukic]

set selected is meant to add a value as selected, and does not clear. This is how it is used internally so this behavior can't be changed.

However clear does appear to have a bug where values set while a label is animating out of view, fail to be set correctly. #2690

[fpayer]

Ok, I would also recommend updating the description of "set selected" in the docs because the description would lead devs to believe it overwrites any previous value.

[jlukic]

I can easily add $('.dropdown').dropdown('set exactly', ['foo', 'baz']); which does clear and then set selected

[fpayer]

Yes, that would be a great solution. Much appreciated.

[jlukic]

I'm also seeing a related bug when using <select> when nothing is selected (via clear), value will report '', which will trigger selection on placeholder value="" and cause the array to have an improper leading comma, i.e. ,VT,VA

[jlukic]

This is actually really fascinating, setting a multi value to '' will then retrieve its value with .val() as [''] if there is a matching option.

[cashsun]

Hi, I think there's a bug in 'set exactly' - the selected labels are not in the same order of passed array.

i.e. $dropdown.dropdown('set exactly', ['d', 'a','c', 'b']);
result: the multiselect shows 'a', 'b', 'c', 'd' as selected, in that order. Which is not exactly 'set exactly'....