Optional parameter in lambda type becomes not optional

[Booksbaum]

export const v: (value?: string) => string;

==>

let [<Import(...)>] v: (string -> string) = jsNative

Parameter isn't optional any more, should be string option.

more examples

export interface I {
    f(value?: string): string;
    readonly v: (value?: string) => string;
    fl(f: (value?: string) => string): void
}

export namespace N {
    function f(value?: string): string;
    const v: (value?: string) => string;
    function fl(f: (value?: string) => string): void

}

export function f(value?: string): string;
export const v: (value?: string) => string;
function fl(f: (value?: string) => string): void

==>

let [<Import("N","module")>] n: N.IExports = jsNative
let [<Import("v","module")>] v: (string -> string) = jsNative

type [<AllowNullLiteral>] IExports =
    abstract f: ?value: string -> string
    abstract fl: f: (string -> string) -> unit

type [<AllowNullLiteral>] I =
    abstract f: ?value: string -> string
    abstract v: (string -> string)
    abstract fl: f: (string -> string) -> unit

module N =

    type [<AllowNullLiteral>] IExports =
        abstract f: ?value: string -> string
        abstract v: (string -> string)
        abstract fl: f: (string -> string) -> unit