Default template model should be struct not array

Postmark returns an error if an array is sent as the template model.
Empty struct works fine. This was causing the following error when
calling `PostmarkHelper.template` with no model :

`{"ErrorCode":403,"Message":"Received valid, but incompatible JSON
input."}`

lib/bamboo/postmark_helper.ex

@@ -24,14 +24,14 @@ defmodule Bamboo.PostmarkHelper do
   Setup Postmark to send emails using a template. Use this in conjuction with
   the template content to offload template rendering to Postmark. The
   template id specified here must match the template id in Postmark.
-  Postmarks's API docs for this can be found [here](https://www.mandrillapp.com/api/docs/messages.JSON.html#method=send-template).
+  Postmarks's API docs for this can be found [here](https://postmarkapp.com/developer/api/templates-api#email-with-template).
 
   ## Example
 
     template(email, "9746128")
     template(email, "9746128", %{"name" => "Name", "content" => "John"})
   """
-  def template(email, template_id, template_model \\ []) do
+  def template(email, template_id, template_model \\ %{}) do
     email
     |> Email.put_private(:template_id, template_id)
     |> Email.put_private(:template_model, template_model)

test/lib/bamboo/postmark_adapter_test.exs

@@ -157,7 +157,7 @@ defmodule Bamboo.PostmarkAdapterTest do
     assert_receive {:fake_postmark, %{params: %{"TemplateId" => template_id,
        "TemplateModel" => template_model}}}
     assert template_id == "hello"
-    assert template_model == []
+    assert template_model == %{}
   end
 
   test "deliver/2 puts template name and content" do