rails/rails — pull request #16635 by adorechic (2014-08-22 11:35:04+00:00)
Title: Extract join values to merge as an original type

```ActiveRecord::QueryMethods#joins!``` receives arguments as variable
length arguments. So that it saves join values as Array.

This affects ```ActiveRecord::Relation::Merger#merge_joins``` which
handles joins by its type. If String joins merged twice, the second
merge process raises ```ActiveRecord::ConfigurationError```