Support soft delete when join.

[c20020207]

When you join right now, you still need to add not is_nil(deleted_at) during join. It is really easy to forget to add as normally you don't need to think about it as it is automatically handled by the library.
Tried to modify the Repo, but not sure if there is any way to do the dynamic binding in the query. (as you can use query.joins to figure out how many join it is, theoretically if there is any way to loop build the query, it might work(
Happy to discuss or implement if anyone have any ideas.