Bugzilla 448776 : WEIGHTEDAVE Expression is not set correctly

The Expression Field in a weighted average aggregation, does not resolve
to row[ÎéÎíxxxxxÎéÎí] like it should, rather just xxxxx.

The workaround is to go through the fx expression builder).
Merge pull request #27 from carlthronson/master

Bugzilla 448776 : WEIGHTEDAVE Expression is not set correctly