Convert strings with boolean values to Boolean type

I got this problem when using several checkboxes in my CRUD forms, when calling $('form').serializeJSON() I was getting the following result:

    Object {
        name: "foobar",
        active: "true",
        targets: Object {
            men: "false",
            women: "true"
        }
    }

So I had to convert the "true" and "false" strings manually before saving the model in the database.
With this PR, serializeJSON will now be able to return the same form as:

    Object {
        name: "foobar",
        active: true,
        targets: Object {
            men: false,
            women: true
        }
    }

p.s.: I'm using node.js as my back-end, not rails as mentioned in the readme. And tried to think why someone would like to read/store the boolean values as string and I just can't find any reason why. If you have a good use-case, please let me know.

See ya.