Additional BiDiBodyLens functions?

[jhult]

Would the following functions be helpful to add to http4k-core? If so, I can submit a PR. If not, I can keep them as extension functions. However, it seems like they might be useful to others (perhaps with some changes to the Exceptions that are thrown, such as using IllegalArgumentException or IllegalStateException).

/**
 * Throws an [InvalidInputException] if the [response][Response] [status][Status] is
 * [400][Status.BAD_REQUEST]. Otherwise, apply the [BiDiBodyLens] to the [response][Response].
 *
 * @see validate
 */
@Throws(InvalidInputException::class)
fun <FINAL> BiDiBodyLens<FINAL>.getOrThrow(response: Response): FINAL {
    if (response.status == Status.BAD_REQUEST) throw InvalidInputException(response.body.toString())
    response.validate()
    return this(response)
}

/**
 * If the [response][Response] is [204][Status.NO_CONTENT], return `null`. Otherwise, apply the
 * [BiDiBodyLens] to the [response][Response].
 *
 * @see validate
 */
fun <FINAL> BiDiBodyLens<FINAL>.optional(response: Response): FINAL? {
    response.validate()
    return if (response.status == Status.NO_CONTENT) null else this(response)
}

/**
 * If the [response][Response] is [204][Status.NO_CONTENT], return an empty [List][emptyList].
 * Otherwise, apply the [BiDiBodyLens] to the [response][Response].
 *
 * @see validate
 */
fun <FINAL> BiDiBodyLens<List<FINAL>>.optional(response: Response): List<FINAL> {
    response.validate()
    return if (response.status == Status.NO_CONTENT) emptyList() else this(response)
}

/**
 * Throws a [ResponseException] if the [Response] [status][Status] is not [200][Status.OK] or
 * [204][Status.NO_CONTENT].
 */
@Throws(ResponseException::class)
fun Response.validate() {
    if (status !in listOf(Status.OK, Status.NO_CONTENT)) throw ResponseException(status)
}