Support passing custom params to Postmark

Postmark's API accepts several parameters that we don't want to directly support to keep the library's API small and clean. This adds support for passing parameters we don't directly support to the request's body.

lib/bamboo/postmark_adapter.ex

@@ -116,8 +116,16 @@ defmodule Bamboo.PostmarkAdapter do
       "Headers": email_headers(email),
       "TrackOpens": true
     }
+    |> add_message_params(email)
   end
 
+  defp add_message_params(params, %{private: %{message_params: message_params}}) do
+    Enum.reduce(message_params, params, fn({key, value}, params) ->
+      Map.put(params, key, value)
+    end)
+  end
+  defp add_message_params(params, _), do: params
+
   defp email_from(email) do
     name = email.from |> elem(0)
     email = email.from |> elem(1)

lib/bamboo/postmark_helper.ex

@@ -35,4 +35,20 @@ defmodule Bamboo.PostmarkHelper do
     |> Email.put_private(:template_id, template_id)
     |> Email.put_private(:template_model, template_model)
   end
+
+  @doc """
+  Put extra message parameters that are used by Postmark. You can set things like TrackOpens or TrackLinks.
+
+  ## Example
+    email
+    |> put_params(email, "TrackLinks", "HtmlAndText")
+    |> put_params(email, "TrackOpens", true)
+  """
+  def put_param(%Email{private: %{message_params: _}} = email, key, value) do
+    put_in(email.private[:message_params][key], value)
+  end
+  def put_param(email, key, value) do
+    email |> Email.put_private(:message_params, %{}) |> put_param(key, value)
+  end
+
 end

test/lib/bamboo/postmark_adapter_test.exs

@@ -158,11 +158,22 @@ defmodule Bamboo.PostmarkAdapterTest do
   end
 
   test "deliver/2 puts tag param" do
-     email = new_email() |> PostmarkHelper.tag("some_tag")
+    email = new_email() |> PostmarkHelper.tag("some_tag")
+    email |> PostmarkAdapter.deliver(@config)
+    assert_receive {:fake_postmark, %{params: %{"Tag" => "some_tag"}}}
+  end
 
-     email |> PostmarkAdapter.deliver(@config)
+  test "deliver/2 puts tracking params" do
+    email = new_email()
+    |> PostmarkHelper.template("hello")
+    |> PostmarkHelper.put_param("TrackOpens", true)
+    |> PostmarkHelper.put_param("TrackLinks", "HtmlOnly")
+
+    email |> PostmarkAdapter.deliver(@config)
 
-     assert_receive {:fake_postmark, %{params: %{"Tag" => "some_tag"}}}
+    assert_receive {:fake_postmark, %{params: %{
+      "TrackLinks" => "HtmlOnly", "TrackOpens" => true, "TemplateId" => "hello"}
+    }}
   end
 
   test "raises if the response is not a success" do