Fix IE bug where it would only trigger when the carrot was at the end of the line

Trying to make this work a little better in IE8. This should fix an error where on keyup, the text it was analyzing was the entire text of the textarea regardless of the location of the carrot; thereby preventing you from using this in the middle of a block of text.

The problem is that IETextarea correctly created a new _getTextFromHeadToCaret function because the original Textarea.getTextFromHeadToCaret() just returns the full text in IE. Not certain why the initial function was intended to be private and not override the one in Textarea. However, now it appears to be working correctly and allows textcomplete in the middle of a block of text.