lib-http: client: Fixed bug in handling of lost connections while returning from...

lib-http: client: Fixed bug in handling of lost connections while returning from another ioloop.

At one instance the http_client_connection_is_ready() function could have destroyed the connection while the caller still depended on it.
Renamed the http_client_connection_is_ready() function to http_client_connection_check_ready().
This now returns -1 when the connection got destroyed. Before it returned a bool that just indicated whether the connection was ready or not.
So, there is no need anymore to preserve a connection reference while calling this function.