Doing `Author.joins(:posts).merge(Post.joins(:comments))` does this
`SELECT ... INNER JOIN posts ON... LEFT OUTER JOIN comments ON...`
instead of doing
`SELECT ... INNER JOIN posts ON... INNER JOIN comments ON...`.

This behavior is unexpected and makes little sense as, basically, doing
`Post.joins(:comments)` means I want posts that have comments. Turning
it to a LEFT JOIN means I want posts and join the comments data, if
any.

We can see this problem directly in the existing tests.
The test_relation_merging_with_merged_joins_as_symbols only does joins
from posts to comments to ratings while the ratings fixture isn't
loaded, but the count is non-zero.