`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.

`type t` = `[`

`|` ``Raw`
`|` ``Normalize`

`]```
`include sig ... end`
`val sexp_of_t : t ‑> Base.Sexp.t`
`val default : t`