Check uniq entity : exclude current entity

[bastos71]

It seems that the checkUniqueEntity validator is missing a big point :

if I'm updating an User entity , leting the unique field login with its original value, the validator will return an error "Value already used" because it does not exclude the current entity from the check action.

I don't know if it has to be done in Fp\JsFormValidatorBundle\Controller\AjaxController::checkUniqueEntityAction(), but at least there should have more data sended through the request to allow us to create our own checkUniqueEntity validator who will handle these cases

See Chapter 3.9 from the documentation for more details.

I will provide a pull request