define the isNumber function used in the input maxlength property shim

I had a bug on IE8 when loading the 'forms' shim on the maxlength property shim. 
The reason was, the 'mexlength''s getter was calling a function 'isNumber' which is not defined in the form-shim-extend2.js file.
I just copy/paste a isNumber function form another js file and this fixes the bug.