Always register the https scheme for https URIs.

This was only being done if the connection was being passed non-null
connection parameters but sometimes, eg just a different port, it needs to be
registered explicitly every time.
Enable multi-threaded use of http connection.

Switch to using a pooled connection manager for the http client. Note that this
does not mean that the library itself is safe for multi-threaded use. More work
will need to be done to be sure of that.