Make Fadd_face_text_property even less destructive

* src/textprop.c (Fadd_face_text_property): We need to be
non-destructive in buffers, too, because face properties may have
been added with `put-text-property' as a list that's been reused.

src/textprop.c

@@ -1351,7 +1351,7 @@ into it.  */)
 			 (NILP (append)
 			  ? TEXT_PROPERTY_PREPEND
 			  : TEXT_PROPERTY_APPEND),
-			 !STRINGP (object));
+			 false);
   return Qnil;
 }