Response headers in the headers toolbar could be incorrect

[a3kov]

Since it's possible to modify headers after toolbar tween finished processing, displayed headers could be incomplete or incorrect. Most common example is when headers are modified by a response callback as using callback is the universal way of working with headers, that works regardless of the way response is generated.
If Pylons/pyramid#2622 is fixed (but that is unlikely as far as I see), this issue could be fixed also by moving the response callback tween below toolbar.