Formats the message body based on the content type.

It uses in order of preference:

1. The content type specified for the plug.
2. The content type specified on the message.
3. The default content type text/plain.

The location of the content type can be changed from content_type to a header with the :header option.

This plug should be used in an outgoing pipeline.

Examples

plug Conduit.Plug.Format
plug Conduit.Plug.Format, content_type: "application/json"
plug Conduit.Plug.Format, header: "Content-Type"

iex> import Conduit.Message
iex> message =
iex>   %Conduit.Message{}
iex>   |> put_body(%{})
iex>   |> Conduit.Plug.Format.run(content_type: "application/json")
iex> message.body
"{}"
iex> message.content_type
"application/json"

iex> import Conduit.Message
iex> message =
iex>   %Conduit.Message{}
iex>   |> put_body(%{})
iex>   |> Conduit.Plug.Format.run(content_type: "application/json", header: "content_type")
iex> message.body
"{}"
iex> get_header(message, "content_type")
"application/json"