abandon_stale.py: send data as a dict, not a string
abandon_stale.py: send data as a dict, not a string

After a recent PR merged on pygerrit2 [1], sending data as a string
no longer works as expected; it must be sent as a dict.

[1] https://github.com/dpursehouse/pygerrit2/pull/10

Change-Id: Idaaff88e9811bf8ca90ef2719f13022ffd09cfad

gerrit-review/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,WANdisco/gerrit,gerrit-review/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,WANdisco/gerrit,WANdisco/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit