[RFR] field.validation(obj) merges obj with the default config

This is a workaround for #134. Now, calling `field.validation({ required: true })` doesn't nullify the `validation.maxlength` setting, and therefore removes the WTF.

But the real solution, in my mind, is to rewrite the ng-minlength and ng-maxlength directives to handle the `undefined` case.
Could you use `!arguments.length` to be consistent with other [getter/setter](https://github.com/marmelab/ng-admin/blob/master/src/javascripts/ng-admin/Main/component/service/config/Configurable.js#L13) ?