<core-menu> item selection should not stay selected when reopened

[0x24a537r9]

It's a bit odd that when I use a twice that has a that the item stays selected after I click it, and when I reopen it, that one is still selected. You can observe this behavior in https://www.polymer-project.org/components/paper-menu-button/demo.html by clicking the first button.

In a regular drop-down field it may make sense to show the currently selected value, but in a (where you're not so much choosing a selection as just choosing one of many actions), showing the previous selection is confusing and strange.

[ootwch]

Workaround: set selected=null after handling the event.

http://stackoverflow.com/a/40357787/1878199