Description:
------------
->fields($expr) fails to fetch keys ["qwe", "asd"] when trying to reach them via "tree.100" path in the following document:

$doc = [
    'pk' => 25,
    'tree' => [
        100 => [
            'qwe' => [
                1
            ],
            'asd' => [
                'a'
            ]
        ],
        200 => 'a',
        'abc' => 999
    ]
];

Test script:
---------------
<?php
$doc = [
    'pk' => 25,
    'tree' => [
        100 => [
            'qwe' => [
                1
            ],
            'asd' => [
                'a'
            ]
        ],
        200 => 'a',
        'abc' => 999
    ]
];
$session = mysql_xdevapi\getSession('mysqlx://root:password@localhost');
$schema = $session->getSchema('test');
$coll = $schema->getCollection(COLL);
$coll->add($doc)->execute();

// Works correctly - fetches tree.100.qwe key.
$expr = mysql_xdevapi\Expression('$.tree."100".qwe');
$result = $coll->find('$.pk=25')->fields($expr)->execute();

// Errors with: mysql_xdevapi\CollectionFind::fields(): Error while parsing, details: CDK Error: After seeing '$.tree."100" AS ', looking at '100': Expected identifier after AS (cdk:8)
$expr = mysql_xdevapi\Expression('$.tree."100"');
$result = $coll->find('$.pk=25')->fields($expr)->execute();

// Errors with: mysql_xdevapi\CollectionFind::fields(): Error while parsing, details: CDK Error: After seeing '$.tree.', looking at '[100] ...': Expected member name or '*' after '.' in a document path (cdk:8)
$expr = mysql_xdevapi\Expression('$.tree.[100]');
$result = $coll->find('$.pk=25')->fields($expr)->execute();

// Errors with: mysql_xdevapi\CollectionFind::fields(): Error while parsing, details: CDK Error: After seeing '$.tree', looking at '.100 A...': Invalid characters in projection specification, only AS <name> allowed after the projection expression (cdk:8)
$expr = mysql_xdevapi\Expression('$.tree.100');
$result = $coll->find('$.pk=25')->fields($expr)->execute();