body_length t is the length of the message body accompanying
t. It is an error to generate a request with a close-delimited message body.
See RFC7230§3.3.3 for more details.
val persistent_connection : ?proxy:bool -> t -> bool
persistent_connection ?proxy t indicates whether the connection for
t can be reused for multiple requests and responses. If the calling code is acting as a proxy, it should pass