add unique alias to custom objects query

[galvani]

Q	A
Bug fix? (use the a.b branch)	[X]

Description:

in case that filter contains two possible values for one filter only one value was used the alias for the union joins was not unique. this fixes it.

Steps to test this PR:

1. Create a report with custom field in dev environment so you can see the SQL
2. filter by same custom fields with two diffetent values like: text = aaa and text = bbb (multiple relations, so it makes sense)
3. check the SQL outputted in the report view.

[driskell]

Might be fixed by #322 in a more complete way