`!` autocomplete does not work anymore

In this commit https://gitlab.com/gitlab-org/gitlab-ce/commit/5c2078838bb9de710f9025513c4b6ec664bba313 we broke ! autocomplete.

The merge_request data source is in snake case, but JS is expecting it camelCase https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/assets/javascripts/gfm_auto_complete.js#L495

The change in this line: https://gitlab.com/gitlab-org/gitlab-ce/commit/5c2078838bb9de710f9025513c4b6ec664bba313#5c81a39dd0a93216b1a0f2dcc14fa6ef4da373eb_423_424 broke it. Because now we require datasource to be defined when it always was undefined