Fix spaceSelectsMatch causing error (fixes #495) (#529)

With `spaceSelectsMatch` enabled, an error `Uncaught TypeError: Cannot
read property '0' of undefined` is sometimes given when attempting to
autocomplete with any of space, tab, or enter.

The cause appears to be that with this option enabled, sometimes this
code path intended to handle KeyboardEvents will be entered again after
handling a KeyboardEvent with another type of event (CustomEvent); this
then later causes this error as `this.current.filteredItems` is now
`undefined`, as the original KeyboardEvent has already been handled.

Checking whether the event looks like a KeyboardEvent, and returning
early if it doesn't, appears to resolve this.

src/TributeEvents.js

@@ -107,7 +107,7 @@ class TributeEvents {
     }
     instance.updateSelection(this);
 
-    if (event.keyCode === 27) return;
+    if (!event.keyCode || event.keyCode === 27) return;
 
     if (!instance.tribute.allowSpaces && instance.tribute.hasTrailingSpace) {
       instance.tribute.hasTrailingSpace = false;