Make some exec_query expectations less fragile between rails versions.

@fbogsany for review

These expectations were failing on rails 4.2.0.rc1 because exec_query took a third parameter and one of the two spaces preceding `WHERE` was removed in the SQL query strings.
Given all 4 examples are identical:
```ruby
AssociatedRecord.unscoped.select(quoted_table_column(AssociatedRecord, :name)).where(id: 1).limit(1).to_sql
```
is it worth factoring this out into a method?

It's a little sad that this is the arel equivalent of `AssociatedRecord.unscoped.where(id: 1).limit(1).pluck(:name)`, which is much nicer to read.