Ignore non-arel nodes in joins_values

[shioyama]

This is a bug with the table backend:

Post.i18n.joins(:translations).where(title: "foo")
#=> NoMethodError: undefined method `left' for :translations:Symbol#=> NoMethodError: undefined method `left' for :translations:Symbol

The joins here is redundant, but the exception is unexpected. We need to filter out non-arel nodes when checking to see if we have already joined translations.