feat(maps): patch concept serialization

[jonathan-casey]

Changes

Patch fix for the serialization of Concepts when used as Map values.

// Valid
{
    $class: 'org.acme.sample.Concepts',
    rolodex: {
        $class: 'org.acme.sample.Rolodex',
        'Dublin': {'$class':'org.acme.sample.Person','name':'Bob'}, // Concept is an Object
        'London': {'$class':'org.acme.sample.Person','name':'Alice'}
    }
};

// Invalid
{
    $class: 'org.acme.sample.Concepts',
    rolodex: {
        $class: 'org.acme.sample.Rolodex',
        'Dublin': '{"$class":"org.acme.sample.Person","name":"Bob"}',  // Concept is not a String
        'London': '{"$class":"org.acme.sample.Person","name":"Alice"}'
    }
};