Add support to Mailgun adapter for using custom vars (#306)

lib/bamboo/adapters/mailgun_adapter.ex

@@ -77,6 +77,7 @@ defmodule Bamboo.MailgunAdapter do
     |> put_html_body(email)
     |> put_text_body(email)
     |> put_headers(email)
+    |> put_custom_vars(email)
     |> filter_non_empty_mailgun_fields
   end
 
@@ -103,16 +104,24 @@ defmodule Bamboo.MailgunAdapter do
 
   defp put_headers(body, %Email{headers: headers}) do
     Enum.reduce(headers, body, fn({key, value}, acc) ->
-      Map.put(acc, :"h:#{key}", value) 
+      Map.put(acc, :"h:#{key}", value)
+    end)
+  end
+
+  defp put_custom_vars(body, %Email{private: private}) do
+    custom_vars = Map.get(private, :mailgun_custom_vars, %{})
+
+    Enum.reduce(custom_vars, body, fn({key, value}, acc) ->
+      Map.put(acc, :"v:#{key}", value)
     end)
   end
 
   @mailgun_message_fields ~w(from to cc bcc subject text html)a
 
   def filter_non_empty_mailgun_fields(map) do
     Enum.filter(map, fn({key, value}) ->
-      # Key is a well known mailgun field or is an header field and its value is not empty
-      (key in @mailgun_message_fields || String.starts_with?(Atom.to_string(key), "h:")) && !(value in [nil, "", []]) 
+      # Key is a well known mailgun field or is an header or custom var field and its value is not empty
+      (key in @mailgun_message_fields || String.starts_with?(Atom.to_string(key), ["h:", "v:"])) && !(value in [nil, "", []])
     end)
   end
 

test/lib/bamboo/adapters/mailgun_adapter_test.exs

@@ -78,14 +78,15 @@ defmodule Bamboo.MailgunAdapterTest do
     assert request_path == "/test.tt/messages"
   end
 
-  test "deliver/2 sends from, subject, text body, html body and headers" do
+  test "deliver/2 sends from, subject, text body, html body, headers and custom vars" do
     email = new_email(
       from: "from@foo.com",
       subject: "My Subject",
       text_body: "TEXT BODY",
       html_body: "HTML BODY",
     )
     |> Email.put_header("X-My-Header", "my_header_value")
+    |> Email.put_private(:mailgun_custom_vars, %{my_custom_var: 42, other_custom_var: 43})
 
     MailgunAdapter.deliver(email, @config)
 
@@ -95,7 +96,9 @@ defmodule Bamboo.MailgunAdapterTest do
     assert params["subject"] == email.subject
     assert params["text"] == email.text_body
     assert params["html"] == email.html_body
-    assert params["h:X-My-Header"] == "my_header_value" 
+    assert params["h:X-My-Header"] == "my_header_value"
+    assert params["v:my_custom_var"] == "42"
+    assert params["v:other_custom_var"] == "43"
 
     hashed_token = Base.encode64("api:" <> @config.api_key)