fix RGBA_ONLY with generic mono

1. Convert MULTIBAND uint/short/etc. as we convert uint8 images. They
   are probably just uint8 that have been cast up in format.

2. Don't check that the image interpretation is a supported colourspace
   before calling vips_colourspace() ... we want to fail with an error
   if it can't be converted, not silently accept the image.

libvips/foreign/foreign.c

@@ -1464,7 +1464,6 @@ vips__foreign_convert_saveable( VipsImage *in, VipsImage **ready,
 	 */
 	if( !coding[VIPS_CODING_RAD] &&
 		in->Bands < 3 &&
-		vips_colourspace_issupported( in ) &&
 		saveable == VIPS_SAVEABLE_RGBA_ONLY ) { 
 		VipsImage *out;
 		VipsInterpretation interpretation;

libvips/iofuncs/header.c

@@ -590,6 +590,9 @@ vips_image_default_interpretation( const VipsImage *image )
 
 	switch( image->BandFmt ) {
 	case VIPS_FORMAT_UCHAR:
+	case VIPS_FORMAT_SHORT:
+	case VIPS_FORMAT_UINT:
+	case VIPS_FORMAT_INT:
 		switch( image->Bands ) {
 		case 1:
 		case 2:
@@ -626,11 +629,6 @@ vips_image_default_interpretation( const VipsImage *image )
 			return( VIPS_INTERPRETATION_MULTIBAND );
 		}
 
-	case VIPS_FORMAT_SHORT:
-	case VIPS_FORMAT_UINT:
-	case VIPS_FORMAT_INT:
-		return( VIPS_INTERPRETATION_MULTIBAND );
-
 	case VIPS_FORMAT_FLOAT:
 	case VIPS_FORMAT_DOUBLE:
 		switch( image->Bands ) {