My router

gistfile1.txt

defmodule FacebookService.Router do
  use FacebookService.Web, :router

  scope "/v1/users", FacebookService do
    get "/", UserIndexController, :index
    get "/:user_id", UserShowController, :show
    put "/:user_id", UserUpdateController, :update
  end
end


defmodule FacebookService.Endpoint do
  use Phoenix.Endpoint, otp_app: :facebook_service
  import Phoenix.Controller, only: [accepts: 2]

  plug Plug.Parsers,
    parsers: [:urlencoded, :multipart, :json],
    pass: ["*/*"],
    json_decoder: Poison

  plug Plug.MethodOverride

  plug :accepts, ["json-api"]
  plug FacebookService.Router
end


My test simply does:
    {:ok, response} = HTTPoison.put(
      "http://localhost:4000/v1/users/1234",
      Poison.encode!(%{"foo" => "bar"}),
      [{"content-type", "application/vnd.api+json"},
       {"accept", "application/vnd.api+json"}])
    IO.inspect response

Running the test produces the following log messages:

No supported media type in accept header in plug :accepts, expected one of ["html"]
#PID<0.469.0> running FacebookService.Endpoint terminated
Server: localhost:4000 (http)
Request: PUT /v1/users/1234
** (exit) an exception was raised:
    ** (Poison.EncodeError) unable to encode value: {:ok, %{"foo" => "bar"}}
        (poison) lib/poison/encoder.ex:213: Poison.Encoder.Any.encode/2
        (poison) lib/poison/encoder.ex:156: anonymous fn/4 in Poison.Encoder.Map.encode/2
        (stdlib) lists.erl:1261: :lists.foldl/3
        (poison) lib/poison/encoder.ex:157: Poison.Encoder.Map.encode/2
        (poison) lib/poison/encoder.ex:156: anonymous fn/4 in Poison.Encoder.Map.encode/2
        (stdlib) lists.erl:1261: :lists.foldl/3
        (poison) lib/poison/encoder.ex:157: Poison.Encoder.Map.encode/2
        (poison) lib/poison/encoder.ex:156: anonymous fn/4 in Poison.Encoder.Map.encode/2
%HTTPoison.Response{body: "",
 headers: [{"connection", "keep-alive"}, {"server", "Cowboy"},
  {"date", "Sat, 29 Aug 2015 06:58:11 GMT"}, {"content-length", "0"},
  {"cache-control", "max-age=0, private, must-revalidate"}], status_code: 406}