fix handling of "squash" param in tiffsave

the deprecated param was not being detected correctly, breaking vips7
compat in some cases

see https://github.com/libvips/libvips/issues/1801

ChangeLog

@@ -6,6 +6,7 @@
 - convert no-profile CMYK to RGB on save [augustocdias]
 - ensure SVG loader skips input with chars outside x09-x7F range [lovell]
 - better mask sizing in gaussmat [johntrunc]
+- fix tiffsave "squash" handling [barryspearce]
 
 24/1/20 started 8.10.0
 - more conformat IIIF output from dzsave [regisrob]

libvips/foreign/tiffsave.c

@@ -406,7 +406,7 @@ vips_foreign_save_tiff_file_build( VipsObject *object )
 
         /* Handle the deprecated squash parameter.
 	 */
-        if( vips_object_argument_isset( object, "squash" ) ) 
+        if( tiff->squash )
 		/* We set that even in the case of LAB to LABQ.
 		 */
 		tiff->bitdepth = 1;