Do *not* report connection reset by peer as an error on an http server

Do *not* report connection reset by peer as an error on an http server
when no request is in progress.

Instead treat it as a normal close of the connection. This error was
introduced yesterday when getting rid of the connection memory
leak. Handling the connection errors as a normal close still makes
sure to clean up so we do not leak memory.

R=sgjesse@google.com
BUG=dartbug.com/6585

Committed: https://code.google.com/p/dart/source/detail?r=14617

[Søren Gjesse, 2012-11-07 10:32:43]

lgtm