Fix existing tag search

Addresses an issue where modifying tag text using the itemText function could result in duplicate tags.

***Example:***
When the below function is active, entering text "test.dot" and "t.estdot" results in the duplicate tag "testdot"

```
$('input').tagsinput({
		itemText: function(item) {
			return this.itemValue(item).replace('.','');
		}
	});
```