Protocols.HTTP.Query: async request fails when keep-alive connection was closed

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=7143

Reported by Martin Karlgren marty@roxen.com

If a Protocols.HTTP.Query instance is used with a keep-alive connection, async_request fails if the remote end closed the connection.

Proposed fix attached.