Fixes HTTPClient::poll crash after connection is self-assigned

(cherry picked from commit 8226cee8d8)

core/io/http_client.cpp

@@ -100,6 +100,10 @@ void HTTPClient::set_connection(const Ref<StreamPeer> &p_connection) {
 
 	ERR_FAIL_COND_MSG(p_connection.is_null(), "Connection is not a reference to a valid StreamPeer object.");
 
+	if (connection == p_connection) {
+		return;
+	}
+
 	close();
 	connection = p_connection;
 	status = STATUS_CONNECTED;