messaging: summary is missing propery decorator

[nikromen]

Fixes #3493

[nikromen]

@abompard per this comment - with the property decorator we should get a reasonable description from summarry, instead of something like:

<bound method BuildChrootStarted.summary of
 BuildChrootStartedV1(id='702cc3bd-450e-44bb-82cf-b92ba4a67e11',
 topic='org.fedoraproject.prod.copr.build.start', body={'user': 'davidsch',
 'copr': 'fixes4bout', 'owner': 'davidsch', 'pkg': None, 'build': 8190255,
 'chroot': 'srpm-builds', 'version': None, 'status': 3, 'ip':
 'xxxxx, 'who':
 'backend.worker-rpm_build_worker:xxxx', 'pid': xxxx, 'what': 'build
 start: user:davidsch copr:fixes4bout pkg:None build:xxxxx
 ip:xxxxx pid:14xxxx'})>

right?

[abompard]

Correct!

[praiskup]

LGTM