when I define doctrine like this:

doctrine:
	orm:
		auto_generate_proxy_classes: '%kernel.debug%'
		default_entity_manager: central
		entity_managers:
			central:
				naming_strategy: doctrine.orm.naming_strategy.underscore
				auto_mapping: true
				metadata_cache_driver: apc

function PayumExtension->prepend

• prepends config to "orm"
• doctrine will create another manager "doctrine.orm.default_entity_manager"
• that manager is used for payum
• that manager will not contain anything I define in yml config (for example metadata_cache_driver I changed to apc)
• payum EM will have default value FOR EVERYTHING (metadata_cache_driver will be array)