[RFR] Fix missing query string after successful login

[makbol]

I tested this solution with my existing project and it worked fine with custom login page and external identity provider. I hope this quick fix will be sufficient.

Fixes #6117

[djhi]

Thanks! Some additional tests validating the search value would be nice.

[makbol]

@djhi Alright, I'll try. Can you give me some tips how I should approach testing that? From what I see you don't have functional tests set. My perspective is testing that would require passing from unlogged state through login process to redirect to app and check if query string stays. Maybe there's a simpler way of testing that?

[makbol]

Okay, functional test added. @djhi let me know if this is sufficient.

[fzaninotto]

Thanks!