Spacebar bug in Firefox

In the [demo](https://davidtheclark.github.io/react-aria-menubutton/demo/), there seems to be a bug when using spacebar to trigger the menu.

### Steps to reproduce:
1. tab to focus the menu trigger "Select a word"
2. spacebar to open menu
3. while the menu is opened and focused, spacebar to close.

**Expected result:** menu closes

**Actual result:** menu closes and opens in succession.

**Tested on:**
- Firefox 60.9.0 (Mac)
- Firefox 45.9.5 (Mac)
- _may affect other Firefox (Mac) versions as well._

### Other Notes

If you change the `Button.tag` to `span`, this works correctly. However, it should be expected that this should work with `button`. An old issue (#76) stated that the `tag` is exposed only for stylistic changes. However, with this bug, it becomes invalid due to this browser quirk.