 
   @doc """
   Shortcut to check if the message is a response.
+
+  ## Examples:
+
+      iex> resp = Sippet.Message.build_response 200
+      iex> resp |> Sippet.Message.response?
+      true
+
   """
   @spec response?(t) :: boolean
   def response?(%__MODULE__{start_line: %StatusLine{}} = _), do: true


  @doc """
  Deletes the first value of `header` in `message`.
  """
  @spec delete_header_front(t, header) :: t
  def delete_header_front(message, header) do


  @doc """
  Returns whether a given `header` exists in the given `message`.

  ## Examples:

      iex> request = Sippet.Message.build_request :invite, "sip:foo@bar.com"
      iex> request = Sippet.Message.put_header request, :cseq, {1, :invite}
      iex> request |> Sippet.Message.has_header?(:cseq)
      true

  """
  @spec has_header?(t, header) :: boolean
  def has_header?(message, header),
