fix(dropdown): use itemToElement for selected label if not null

[jendowns]

Closes #4913

As described in #4913, if a someone utilizes itemToElement to render a custom component inside dropdown items, that custom component isn't used when you select a dropdown item. Instead, a selected dropdown item is made into a string regardless of implementation.

You can demonstrate this here: http://react.carbondesignsystem.com/?path=/story/dropdown--items-as-components
(Select one of the custom options with blue text, and you will see that the blue option number isn't shown as "selected" -- the option text is all black, meaning the custom component for the option was dropped)

So this PR changes the "selected item" label rendering logic to include itemToElement if it is used.

Changelog

Changed

• use itemToElement for "selected" dropdown item label, if itemToElement is not null... otherwise, do what was done before with rendering as a string.

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[asudoh]

LGTM 👍 - Thanks @jendowns!

[abbeyhrt]

🎉 Thanks @jendowns!

[joshblack]

I'm a little confused, I had interpreted that itemToElement was meant for rendering an option in the list of options, is that not the case?

[jendowns]

@joshblack -- when you say "rendering an option" do you mean using a custom component as an option, rather than just text? That's what my interpretation of it is. So if you put blue text or something in the list of options and then select that option, now that "selected" option will be blue like it was in the list. (Previously the selected option was just rendered as a string regardless of if you did something custom)

[joshblack]

@jendowns yeah, I think it should be meant only for rendering an option in a list. If we support custom rendering for the value on top of the field, we won't be able to control things like contrast or could end up with violations if people decide to put things other than text in that location. Could easily be misreading though, let me know.

[jendowns]

Oh yeah that's a good point @joshblack -- do you think that same concern should apply to the list of options as well, which could for example have a custom option that doesn't meet contrast requirements, etc.?

Also would you prefer I revert / undo this?