You can put values for a property in the query string of a new
object definition.

E.G. https://issues.roundup-tracker.org/issue?
@template=item&type=behavior

However if the type property had a default_value defined in the schema,
the value specified in the url is ignored.

If there is a value in the URL, it should be used. If the value
is unset after looking at all sources for the value, then use
the default_value.

Also if an invalid value is supplied in the URL for a property
without a default_value, we get a traceback. If the URL value is
invalid, it should just be ignored.