Maarten/headers through options

lib/icalendar.ex

@@ -55,8 +55,9 @@ defimpl ICalendar.Serialize, for: ICalendar do
     vendor = Keyword.get(options, :vendor, "Elixir ICalendar")
     name = Keyword.get(options, :name)
     id = Keyword.get(options, :id)
+    headers = Keyword.get(options, :headers, [])
 
-    [vendor: vendor, name: name, id: id]
+    [vendor: vendor, name: name, id: id] ++ headers
     |> Enum.reject(fn {_, v} -> is_nil(v) or v == "" end)
     |> Enum.map(&attribute/1)
     |> Enum.join("\n")
@@ -65,4 +66,5 @@ defimpl ICalendar.Serialize, for: ICalendar do
   defp attribute({:vendor, vendor}), do: "PRODID:-//Elixir ICalendar//#{vendor}//EN"
   defp attribute({:name, name}), do: "X-WR-CALNAME:#{name}"
   defp attribute({:id, id}), do: "X-WR-RELCALID:#{id}"
+  defp attribute({key, value}), do: "#{key}:#{value}"
 end

lib/icalendar/serialize.ex

@@ -10,6 +10,8 @@ defprotocol ICalendar.Serialize do
       `X-WR-CALNAME:Calendar Name`.
     * `id` a string containing the calendar's id. Will produce
       `X-WR-RELCALID:123456`.
+    * `headers` a keyword list containing the headers to
+      be placed in the  calendar header: `[{"Method", "Request"}]`
   """
   def to_ics(data, options \\ [])
 end

test/icalendar_test.exs

@@ -27,6 +27,19 @@ defmodule ICalendarTest do
            """
   end
 
+  test "ICalendar.to_ics/1 of empty calendar with custom headers" do
+    ics = %ICalendar{} |> ICalendar.to_ics(headers: [{"METHOD", "REQUEST"}])
+
+    assert ics == """
+           BEGIN:VCALENDAR
+           CALSCALE:GREGORIAN
+           VERSION:2.0
+           PRODID:-//Elixir ICalendar//Elixir ICalendar//EN
+           METHOD:REQUEST
+           END:VCALENDAR
+           """
+  end
+
   test "ICalendar.to_ics/1 of a calendar with an event, as in README" do
     events = [
       %ICalendar.Event{