SwiftでrailsのwebAPIを使ったアプリの開発でPOSTできない

Question

###発生している問題・エラーメッセージ
フロントサイドをswift,バックエンドをrailsを使い、APIサーバーを使う簡易的なアプリを作ろうとしているのですが、swift側からPOSTをリクエストすると
param is missing or the value is empty: product
のエラーが出ます。

なにが悪いのでしょう？
初心者なので、丁寧に教えていただけると非常に助かります><

###ソースコード

class ProductsController < ApplicationController
  before_action :set_product, only: [:show, :edit, :update, :destroy]
  skip_before_filter :verify_authenticity_token, only: :create

  # GET /products
  # GET /products.json
  def index
    @products = Product.all
    respond_to do |format|
      format.html # => 通常のURLの場合、index.html.erb が返される
      format.json { render json: @products } # URLが.jsonの場合、@products.to_json が返される
    end
  end

  # GET /products/1
  # GET /products/1.json
  def show
  end

  # GET /products/new
  def new
    @product = Product.new
  end

  # GET /products/1/edit
  def edit
  end

  # POST /products
  # POST /products.json
  def create
    @product = Product.new(product_params)

    respond_to do |format|
      if @product.save
        format.html { redirect_to @product, notice: 'Product was successfully created.' }
        format.json { render :json, @product }
      else
        format.html { render :new }
        format.json { render json: @product.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /products/1
  # PATCH/PUT /products/1.json
  def update
    respond_to do |format|
      if @product.update(product_params)
        format.html { redirect_to @product, notice: 'Product was successfully updated.' }
        format.json { render :show, status: :ok, location: @product }
      else
        format.html { render :edit }
        format.json { render json: @product.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /products/1
  # DELETE /products/1.json
  def destroy
    @product.destroy
    respond_to do |format|
      format.html { redirect_to products_url, notice: 'Product was successfully destroyed.' }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_product
      @product = Product.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def product_params
      params.require(:product).permit(:name, :price, :category_id)
    end
end

mport UIKit
import Alamofire

class ViewController: UIViewController {

    @IBOutlet weak var priceText: UITextField!
    @IBOutlet weak var nameText: UITextField!
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    @IBAction func saveBtnTap(sender: AnyObject) {
        
        let memo = Memo()
        memo.name = nameText.text
        memo.price = Int(priceText.text!)
        memo.category_id = 1
        
        StockMemos.postMemo(memo)
    }

}

import UIKit
import Alamofire

class StockMemos: NSObject {
    
    class func postMemo(memo: Memo) {
        
        var params: [String: AnyObject] = [
            "name": memo.name!,
            "price":memo.price!,
            "category_id":memo.category_id!
        ]
        
        // HTTP通信
        Alamofire.request(.POST, "http://localhost:3000/products.json", parameters: params, encoding: .URL).responseJSON(completionHandler: { response in
            if response.result.isSuccess{
                print("成功しました")
            }
            
                  })
        
        }
        
    }

import UIKit

class Memo: NSObject {
    var name:String?
    var price:Int?
    var category_id:Int?
    }

###補足情報(言語/FW/ツール等のバージョンなど)
Swift 2.1
Xcode 7.1.1
ruby 2.1.3
rails 4.2.5.1