message: Make NamedFieldDocument deserializable
diff: use crate::schema::Value;
  -use serde::Serialize;
  +use serde::{Deserialize, Serialize};
  use std::collections::BTreeMap;
  /// Internal representation of a document used for JSON
  @@ -8,5 +8,5 @@ use std::collections::BTreeMap;
  /// A `NamedFieldDocument` is a simple representation of a document
  /// as a `BTreeMap<String, Vec<Value>>`.
  ///
  -#[derive(Serialize)]
  +#[derive(Debug, Deserialize, Serialize)]
  pub struct NamedFieldDocument(pub BTreeMap<String, Vec<Value>>);
  