Explicit placement of hidden fields with nested models

When using fields_for to build DOM structure for nested models, the id
hidden field gets emitted implicitly and the developer has no control
over the placement which can result in invalid HTML. I've submitted
the patch below which adds a method, #hidden_fields, to FormBuilder to
explicitly place the hidden fields. If this methods gets called in the
#fields_for block the FormBuilder won't implicitly emit the hidden