fix validation of user and password for Nette\DI\Statement

[krajcikondra]

Hi, my config looks like this:

rabbitmq:

connection:
	host: @envReader::get(RABBIT_MQ_HOST)
	port: @envReader::get(RABBIT_MQ_PORT)
	user: @envReader::get(RABBIT_MQ_USER)
	password: @envReader::get(RABBIT_MQ_PASSWORD)
	vhost: @envReader::get(RABBIT_MQ_VHOST)

In method \Kdyby\RabbitMq\DI\RabbitMqExtension::loadConnections is validation expecting string in user and password.

If we use service envReader for load environmental variable, then to the validation is passed Nette\DI\Statement instead string and application failed.