types: add Headers.singleton

types/headers.ml

@@ -38,6 +38,8 @@ type t = (name * value) list
 
 let empty : t = []
 
+let singleton nv = [ nv ]
+
 let of_rev_list t = t
 let of_list t = of_rev_list (List.rev t)
 let to_rev_list t = t

types/headers.mli

@@ -68,6 +68,7 @@ type value = string
 val ci_equal : string -> string -> bool
 
 val empty : t
+val singleton : (name * value) -> t
 
 val of_list     : (name * value) list -> t
 val of_rev_list : (name * value) list -> t