Merge of r3964, r3977, r3978:

Merge of r3964, r3977, r3978:

Proxy related fixes:

*) finalizing with rc == 0 in unbuffered proxy mode caused nginx to wait
   for another send_timeout before actually closing client's connection
   if client timed out while still talking to upstream server
*) update r3945 with more descriptive error message
*) test length of proxy_pass with variables;
   patch by Lanshun Zhou