Has many attributes fix & recursively call to_params

Currently Her doesn't work very well with rails forms and has_many relations. For example:

 class Product
    include Her::Model
    has_many :items
   attributes :name
end

class Items
  include Her::Model
  attributes :colors
end

The params to ProductController's create action might look like:
{ ...
  "product"=>{
    "name"=>"test product",
    "items_attributes"=>{
      "1389124990194"=>{
        "color"=>"red",
      }
    }
  }
}
But, her expects items_attributes to be an array. This patch allows items_attributes to be a hash that is correctly converted to an array.

The second thing this patch does is recursively call to_params on has_many relations during save. Currently it appears that rather than recursively call to_params, objects in a has_many relation are just having to_s called which is rarely the desired behavior.