Webclient: no trailing "?" in URLs (#483).

Requests issued by the Webclient no longer have an superfluous "?" character at the end.

BREAKING CHANGE

REST Client API:
The `RequestParameter`'s `to_query_string` method is now responsible for generating the "?" character.
Formerly this was the responsibility of the plugin developer.

If your plugin makes use of
    dm4c.restc.createRequestParameter()
you must follow the new `to_query_string` semantics when constructing URLs.

Close ticket 483.