It is possible to enter an invalid value in an autocomplete combobox

[shawninder]

Reproducing the problem:

1. Go to ​http://jqueryui.com/demos/autocomplete/#combobox
2. Click in the input field
3. Write a
4. Click on the arrow beside the input field

What should happen: The list of options should appear under the input field. Choosing one of these options should select it, and clicking somewhere else on the page should empty the input field.

What does happen: The list of choices does not appear and the value of the input field remains unchanged. In theory, one could now proceed to click a submit button thinking that his invalid value will be submitted.

[Scott González]

Duplicate of #7197.
Works fine in master.