ClientProxy detection fix

[dormando]

> I just independently arrived at the same conclusion, so I think that _is_
> the correct fix.
> 
> I think what happens is a request comes in to a service selector, goes to
> proxy, then http_response_sent happens, gets blessed back to base, then
> backend times out or whatever, and boom.
> 
> The hard thing here will be writing a test case for this.
> 
> - Brad
> 

Ahhah, I got halfway there! While tracing code that was my hunch of the 
bug. I couldn't figure out how to make the code actually *go* that way 
though. I'll give it some thought later and ping back if I can think of 
a test case, there's another very minor perlbal bug I found that might 
be hitting the same path.

-Dormando