Add the convenience function Header.is_empty

Author: dinosaure

cohttp/src/header.ml

@@ -50,6 +50,8 @@ let is_header_with_list_value =
 let init () =
   StringMap.empty
 
+let is_empty x = StringMap.is_empty x
+
 let init_with k v =
   StringMap.singleton (LString.of_string k) [v]
 

cohttp/src/header.mli

@@ -22,6 +22,9 @@ type t [@@deriving sexp]
 (** Construct a fresh, empty map of HTTP headers *)
 val init : unit -> t
 
+(** Test whether a map is empty or not. *)
+val is_empty : t -> unit
+
 (** Construct a fresh map of HTTP headers with a single key and value entry *)
 val init_with  : string -> string -> t