Multi-value variables incorrectly handled

the library incorrectly handles multiple occurrences of a parameter.

For example:  http://foo.bar.net/path?a=123&a=456

Request being a such a thin wrapper over dict is problematic here.  At the very least is needs to be a dictionary of lists (a list of tuples is a bit harder to work with).  Which would require a custom urlencode and a custom update() method.
