 
   ## Examples:
 
-      iex> request = Sippet.Message.build_request :invite, "sip:foo@bar.com"
-      iex> request = Sippet.Message.put_header_front(request, :content_language,
-      ...>   "de-DE")
-      iex> request |> Sippet.Message.put_header_front(:content_language,
-      ...>   "en-US")
+      iex> Sippet.Message.build_request(:invite, "sip:foo@bar.com")
+      ...> |> Sippet.Message.put_header_front(:content_language, "de-DE")
+      ...> |> Sippet.Message.put_header_front(:content_language, "en-US")
       %Sippet.Message{body: nil, headers: %{content_language: ["en-US", "de-DE"]},
        start_line: %Sippet.Message.RequestLine{method: :invite,
         request_uri: %Sippet.URI{authority: "foo@bar.com", headers: nil,


      iex> Sippet.Message.to_method("INVITE")
      :invite
      iex> Sippet.Message.to_method("InViTe")
      :invite
      iex> Sippet.Message.to_method("aaa")
      "AAA"
  """
  @spec to_method(String.t) :: method
  def to_method(string), do: string_to_method(string |> String.upcase())


  ## Examples:

      iex> Sippet.Message.build_request(:invite, "sip:foo@bar.com")
      ...> |> Sippet.Message.put_new_header(:max_forwards, 70)
      ...> |> Sippet.Message.put_new_header(:max_forwards, 1)
      %Sippet.Message{body: nil, headers: %{max_forwards: 70},
       start_line: %Sippet.Message.RequestLine{method: :invite,
        request_uri: %Sippet.URI{authority: "foo@bar.com", headers: nil,
