gst-inspect: print current value as default value

gst-inspect: print current value as default value

Instead of printing separate 'Current' and 'Default' values
(the former obtained via g_object_get() and the latter from
the property GParamSpec), simply print the Current value as
the Default value. This is the right thing to do for almost
all elements and avoids confusion if a subclass of a base
class chooses a different default than the base class.