Collection name can be either a string or array:
- if string considered as the name of the collection inside the default database.
- if array - first element considered as the name of the database, second - as
name of collection inside that database
By default this method returns the class name as the collection name by calling [[Inflector::camel2id()]].
For example, 'Customer' becomes 'customer', and 'OrderItem' becomes
'order_item'. You may override this method if the collection is not named after this convention.