fixed notice "Undefined index: field_format"

http://book.cakephp.org/2.0/en/models/data-validation.html#adding-your-own-validation-methods
fixed notice "Undefined index: custom_field_id"
removed "&"
fixed form url

before this fix, url after submitting like "/custom_fields/edit/1/id:1".
sometimes $this->data['CustomField'] does not exist
Merge pull request #194 from okonomi/custom-field-edit-problem-fix

Fixed custom field edit problem