message: Inserting null causes runtime error
diff: @@ -13,6 +13,7 @@ use cdrs::query::QueryParamsBuilder;
  use cdrs::types::value::Value;
  use std::panic;
  use cdrs::types::IntoRustByName;
  +use cdrs::types::map::Map;
  // default credentials
  @@ -30,6 +31,7 @@ struct User {
  pub gender: String,
  pub session_token: String,
  pub state: String,
  + pub some_map: Option<Map>
  }
  @@ -105,9 +107,10 @@ fn read_from_user_table() {
  println!("read_from_user_table");
  let ctx = TestContext::new();
  let mut session = ctx.client.start(Compression::None).unwrap();
  - let select_query = QueryBuilder::new("SELECT user_name, password, gender, session_token, state
  - FROM user_keyspace.users")
  - .finalize();
  + let select_query = QueryBuilder::new("\
  + SELECT user_name, password, gender, session_token, state, some_map \
  + FROM user_keyspace.users"
  + ).finalize();
  let query_op = session.query(select_query, true, true);
  @@ -138,6 +141,10 @@ fn read_from_user_table() {
  user.state = state;
  }
  + if let Some(Ok(m)) = row.get_by_name("some_map") {
  + user.some_map = Some(m);
  + }
  +
  user
  })
  .collect();
  @@ -191,7 +198,8 @@ fn create_table() {
  gender varchar,
  session_token varchar,
  state varchar,
  - birth_year bigint
  + birth_year bigint,
  + some_map map<text, text>
  );";
  let create_table_query = QueryBuilder::new(create_table_cql)
  