message: Pass sql_types test, migrate update.rs to support string type
diff: @@ -87,6 +87,10 @@ impl<'a> Update<'a> {
  value.clone_by(ast_value)
  }
  + Expr::Identifier(ident) => ident.quote_style.map_or_else(
  + || Err(UpdateError::ExpressionNotSupported(column.value.clone()).into()),
  + |_| Ok(Value::String(ident.value.clone())),
  + ),
  _ => Err(UpdateError::ExpressionNotSupported(column.value.clone()).into()),
  }
  