Whitespace.t specifies how to handle header values. It is used in two contexts:
1) Transport: Specify how to turn a string into a header value. `Normalize will add
the necessary for transport.
2) Processing: Specify how to turn a header value into a string. `Normalize will remove
all leading and trailing whitespace on each line in order to cleanly process the
value.
include sig ... endval t_of_sexp : Sexplib.Sexp.t ‑> tval __t_of_sexp__ : Sexplib.Sexp.t ‑> tval sexp_of_t : t ‑> Sexplib.Sexp.tval default : t