Return trashed models if told so on non-eager queries (fixes #4870)

This fixes #4870 bug, when ```morphTo()->withTrashed()``` has no effect when querying polymorphic associations non-eagerly. With tests and everything.
@JosephSilber thanks for the comment, I've removed the ```else``` block and force-pushed, since this change doesn't deserve a separate commit. Also cleared up a bit the commit message.

We still need to return the unmodified query though, otherwise it breaks tests with ```Fatal error: Call to a member function whereIn() on a non-object``` on line 166 at ```framework/src/Illuminate/Database/Eloquent/Relations/MorphTo.php``` (at least with PHP 5.4.24).