Fixed entity in helpers

## Problem

Schema are used as array inside helpers, throwing Exception when fields are Cake/Database/Schema/Table Object

## Solution

if fields are not an array but an object, extract columns using columns() method from Cake/Database/Schema/Table class

## Note

I wasn't able to write a test to show the problem and i'm not sure my fix is the good way of doing thing (I didn't get all the logic here). 

I also edited the FormHelper but i'm not sure the code is finished so feel free to do a better fix ^^