Phoenix.ConnTest appears not to pass request body to controller

conntest_example.ex

# router.ex
defmodule Myapp.Router do
  use Myapp.Web, :router

  pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/", Myapp do
    pipe_through :api
    post "/", PageController, :post
  end
end

# page_controller.ex
defmodule Myapp.PageController do
  use Myapp.Web, :controller

  def post(conn, params) do
    {:ok, body, conn} = conn |> Plug.Conn.read_body
    conn |> json(%{"received_body": body})
  end
end

# page_controller_test.exs
defmodule Myapp.PageControllerTest do
  use Myapp.ConnCase

  @endpoint Myapp.Endpoint

  test "POST /" do
    body_obj = %{"ok" => true}
    resp = conn
           |> put_req_header("content-type", "application/json")
           |> post("/", Poison.encode!(body_obj))

    %{"received_body" => received_obj} = resp.resp_body |> Poison.decode!
    assert(body_obj == received_obj) # this test fails because received_obj == ""
  end
end

Pro