Block: Explicit convert Message to string

Block: Explicit convert Message to string

Without a converting method like ->parse(), ->escaped() or ->text() the
method __toString() is used which is a equivalent to ->parse().

This change switches from ->parse() to ->text() and let Html::element do
the HTML escaping, because the message 'autoblockid' is not needed to
parse.

Change-Id: I653906bdca0864989ffed5960505b96053456066