Fix response leak that can be caused by an exception during redirect

[idelpivnitskiy]

Motivation:

RedirectConfig accepts user-defined functions and it's possible that some of them can throw. RedirectSingle expects it and applies a try-catch block, but we forgot to drain the response payload body before propagating the error.

Modifications:

• Drain response payload before propagating an exception during redirect processing;
• Enhance tests to validate expected behavior;

Result:

We do not leave undrained response payload if redirect fails with an exception.