XMLRPC: getTaxonomies default shows all taxonomies

[markoheijnen]

When using the new method wp.getTaxonomies it default shows all the taxonomies.
With the submitted patch you can control it trough the fourth argument so you can always show all of them.

[markoheijnen]

New unit test

[nacin]

Is public => true the desired default filter? show_ui => true might make more sense for post types, maybe not for taxonomies. Should we introduce a new flag for exposing via XML-RPC flag for both post types and taxonomies?

[markoheijnen]

Yes, that indeed makes more sense.

[maxcutler]

Taken care of in #20566.