paper-autocomplete: suggestions shouldn't close when hitting enter

For a paper-autocomplete with `required`, when you type a **valid** value and enter, the suggestions is closed and the input is in error with "This is required.".

This is kinda weird from a user perspective, I just type the value.

In angular:

https://material.angularjs.org/latest/demo/autocomplete#floating-label

They do not close the suggestions when typing enter which make it more obvious that we want the user to pick in one of the suggestions.