Fixing error when free tagging

There is a bug when you use `allowFreeTagging`.

Reproduce steps:
1. Use the demo page in version 1.6.1 and scroll to "Free Tagging" at the bottom
2. Start typing something that will match a search (ex: 'a')
3. Pause to let run_search() get called and populate the drop-down
4. Continue typing to a point where it no longer matches an item in the drop-down (ex: 'abc')
5. Hit enter
6. Notice that the item doesn't get populated in the window

What's happening is `selected_dropdown_item` get's set during the `run_search()` -> `populate_dropdown()` -> `selected_dropdown_item()` sequence where it matched a search item when you first started typing. The problem is `selected_dropdown_item` never gets cleared when you continue typing after it no longer matches any items in the dropdown. This breaks the code in the keydown callback. It incorrectly thinks there is a `selected_dropdown_item` (line 343) instead of falling into the else statement that checks for `allowFreeTagging` (line 347).