[HTMLSelect] fix option labels display & use value as label if not specified

[jaamison]

Changes proposed in this pull request:

This PR addresses 2 related but different issues with the core/HTMLSelect component:

1) Options' labels are not being rendered

I first noticed this on the Popover example page:

Apparently, React (16.2 at least) does not respect use of the label HTML attribute in lieu of child text nodes for <option> elements (upstream bug?).

This PR reverts to specifying labels for <option> items as inner HTML, rather than specifying them via the element's label attribute, which appears to display correctly:

2) If consumer supplies an array of {value, label} plain objects as options={...} , the component does not fallback to using the value of value as the label in cases where label is either not supplied or is falsy.

I understand that it might seem logical to put the burden of ensuring proper argument syntax on the consumer, but I can foresee cases where this fallback logic would avoid unpleasant surprises, even for consumers who are aware of the proper syntax.

For instance, if consumer supplies:

options={ [{value:1, label:"one"}, {value:2}, {value:3, label:"three"}, {value:4, label:undefined}] }

the the current behavior would be to render options 2 & 4 as empty. The logic in this PR would at least render "2" & "4" respectively.

[giladgray]

this is wholly too much code. the existing map is almost enough:

        const optionChildren = options.map(option => {
            const { value, label }: IHTMLOptionProps = typeof option === "object" ? option : { value: option };
            return (
                <option key={value} value={value}>
                    {label || value}
                </option>
            );
        });

[giladgray]

this function is too similar to lines 85-88. DRY please.

[giladgray]

pushed my code above to #2681

[giladgray]

@jaamison thanks for pushing this up and pointing out these issues! i fixed the component myself in the interest of time -- ready to cut the patch release asap.

[jaamison]

awesome, much more concise