return default value for unset props

we used to issue a warning and return early, but this can leave garbage
in the *value pointer, I think

ruby gobject-introspection will walk object props during GC and can see
state inbetween init and build when not all objects have been given a
value ... we don't want these warnings

ChangeLog

@@ -1,5 +1,6 @@
 7/10/15 started 8.2.0
 - added im_bufmagick2vips(), a vips7 wrapper for magick load from buffer
+- fetch unset property now returns default value rather than warning
 
 7/5/15 started 8.1.1
 - oop, vips-8.0 should be vips-8.1, thanks Danilo

doc/libvips-docs.xml

@@ -9,11 +9,7 @@
   <bookinfo>
     <title>VIPS Reference Manual</title>
     <releaseinfo>
-<<<<<<< HEAD
       For VIPS 8.2.0.
-=======
-      For VIPS 8.1.1.
->>>>>>> 8.1
       The latest version of this documentation can be found on the 
       <ulink role="online-location"
         url="http://www.vips.ecs.soton.ac.uk/index.php?title=Documentation">VIPS website</ulink>.

libvips/iofuncs/object.c

@@ -1315,11 +1315,12 @@ vips_object_get_property( GObject *gobject,
 	g_assert( ((VipsArgument *) argument_class)->pspec == pspec );
 
 	if( !argument_instance->assigned ) {
-		g_warning( "%s: %s.%s attempt to read unset %s property",
-			G_STRLOC,
-			G_OBJECT_TYPE_NAME( gobject ),
-			g_param_spec_get_name( pspec ),
-			g_type_name( G_PARAM_SPEC_VALUE_TYPE( pspec ) ) );
+		/* Set the value to the default. Things like Ruby
+		 * gobject-introspection will walk objects during GC, and we
+		 * can find ourselves fetching object vaklues between init and
+		 * build.
+		 */
+		g_param_value_set_default( pspec, value );
 		return;
 	}