set_force_http1
Description:
Sets whether the messages created through the this using new_message or new_message_from_uri should force use of the HTTP/1, instead of trying HTTP/2 and fallback to HTTP/1, when the newer version cannot be used.
The property has no effect when 
        util_get_force_http1_supported returns false.
Parameters:
| this | an SoupSession | 
| force_http1 | the value to set |