Null tag added to list when tagging-token is pressed fast

[thielmal]

Bug description:

As described in #520 a null-tag is still added to the list, when a tagging token (ENTER or TAB) is pressed very fast after any input (Before the "(custom 'new' label)" appears in the options).

Link to minimally-working plunker that reproduces the issue:

The issue can be seen in the tagging demo: http://angular-ui.github.io/ui-select/demo-tagging.html

To reproduce, enter any text in the first ui-select and press ENTER or TAB immediately.

ui-select version: 19.6
angular version: 1.5.0