[ComboBox] Unexpected behavior when using combobox inside form

I’m running into an issue with the `EuiComboBox` inside a `<form>`, where if I hit “enter” to select an option, it also tries to submit the form. 

I believe this section of code needs to be updated to also call `e.preventDefault()` https://github.com/elastic/eui/blob/master/src/components/combo_box/combo_box.js#L389
