MathQuill always set `avgChWidth` to `Infinity`

When reading line 199 of text.ts of MathQuill,
I think this.text is a function defined at line 80, and the length of a function provides the number of arguments passed to the function, so this.text.length is always zero, and the division by zero sets avgChWidth to Infinity.

// insert cursor at approx position in DOMTextNode
var avgChWidth = this.jQ.width() / this.text.length;

I think it should be textPc.text.length to get the length of the text content.