Make jQuery.isNumeric consistent

[timmywil]

Some people understand isNumeric as, "can I do math on this thing?" Others are asking, "Is this actually a number?" Now that we have no internal usage of jQuery.isNumeric, I think we can choose one or the other and update the documentation. Right now, it is inconsistent and a confusing confluence of the two.

[timmywil]

@dmethvin, @gibson042 and I propose allowing all input and returning true for strings and numbers – whether primitive or object – that can be numeric.

[timmywil]

jQuery.isNumeric = function( input ) {
  var type = jQuery.type( input );
  return ( type === "number" || type === "string" ) && ( input - parseFloat( input ) + 1 ) >= 0;
};

[dmethvin]

I added the behavior change tag since there's that one strange case with a custom .toString() that we will no longer handle. It can just be mentioned in the release notes.