Improve how HttpConnection detects a closed channel when taking/returning a connection to the pool

There is a race condition when the server closes a connection at roughly the same time that the client takes it of the pool. This is particularly observable when the server/proxy closes the connection just after returning 401/407, but can also be observed in other circumstances.
The detection of a channel state could be improved by attempting to read one character off the connection just after taking the connection off the pool.