BUG/MEDIUM: mailers: Set the object type for check attached to an email alert

BUG/MEDIUM: mailers: Set the object type for check attached to an email alert

The health-check attached to an email alert has no type. It is unexpected,
and since the 2.6, it is important because we rely on it to know the
application type in front of a connection at the stream-connector
level. Because the object type is not set, the SE descriptor is not properly
initialized, leading to a segfault when a connection to the SMTP server is
established.

This patch must be backported to 2.6 and may be backported as far as
2.0. However, it is only an issue for the 2.6 and upper.