HttpRequest: close the connection on Stop()

When calling Stop(), we expect the request thread to exit as soon as
possible. Closing the connection unlocks it from any blocking read() or
write(), avoiding some lockup situations.

headers/os/net/HttpRequest.h

@@ -47,6 +47,7 @@ public:
 									const ssize_t size = -1);
             void                AdoptHeaders(BHttpHeaders* const headers);
 
+			status_t			Stop();
 			const BUrlResult&	Result() const;
 			const char*			StatusString(status_t threadStatus) const;