[5.2] Fix morphTo without SoftDeletes

[acasar]

The whole approach with the cloned query sadly doesn't work in some edge cases. Instead we need to take a similar approach approach as we do with "has" queries (merging where clauses manually).

I added one more test to expose the issue (#13762) and refactored the implementation.

@taylorotwell @phroggyy

[acasar]

I removed this helper method since it's not needed any more. It was added here #13784 and hasn't been tagged yet.

[phroggyy]

Shouldn't we merge more than just wheres? There are lots of types here, such as joins

[acasar]

We currently only merge wheres in has queries too, so it should be enough for now.

[tiran133]

Thank you this fixes the problem for me.

[phroggyy]

@acasar: why not just go through the keys of the bindings array? Basically

foreach($this->query->getBindings() as $type => $binding) {
    $query->{$type} = $this->query->{$type}
}

Or similar

[acasar]

@phroggyy Can't do it like you're suggesting since binding keys are not named the same as query parameters. As I said, for now I took this function (for has queries) and adjusted it accordingly: https://github.com/laravel/framework/blob/5.2/src/Illuminate/Database/Eloquent/Builder.php#L989

If we decide to merge other clauses, we should do it in both places with another PR, since it's actually unrelated issue.