Work with soft-deleted causers

[EthicalJobsJesse]

At the moment if a causer is soft-deleted you can't see there records. Adding this config allows you to resolve soft-deleted causers as well

[Gummibeer]

Hey,
thanks for this PR. But the test cases for this change are missing.
And in #456 we've already discussed some issues with the most common implementations. The last comment by @LeonAlvarez looks promising. Haven't tested it but could be that removing the scope in a whole could prevent conflicts if the relationship returns a mix of soft deletable and normal models.

At the end I won't merge an implementation that doesn't work with the whole morphable relationship.

[Gummibeer]

PR doesn't contain unittests that prove that the implementation works with mixed relationships.