Replace the use of OBJECT (#6900)

Rf_isObject has existed for a long time, so no compatibility problems
envisioned.

Author: aitap

src/assign.c

@@ -1157,7 +1157,7 @@ const char *memrecycle(const SEXP target, const SEXP where, const int start, con
           }
           break;
         }
-        if (OBJECT(source) && getAttrib(source, R_ClassSymbol)!=R_NilValue) {
+        if (isObject(source)) {
           // otherwise coerceVector doesn't call the as.character method for Date, IDate, integer64, nanotime, etc; PR#5189
           // this if() is to save the overhead of the R call eval() when we know there can be no method
           source = PROTECT(eval(PROTECT(lang2(sym_as_character, source)), R_GlobalEnv)); protecti+=2;

src/dogroups.c

@@ -516,10 +516,10 @@ SEXP keepattr(SEXP to, SEXP from)
   SET_ATTRIB(to, ATTRIB(from));
   if (isS4(from)) {
     to = PROTECT(asS4(to, TRUE, 1));
-    SET_OBJECT(to, OBJECT(from));
+    SET_OBJECT(to, isObject(from));
     UNPROTECT(1);
   } else {
-    SET_OBJECT(to, OBJECT(from));
+    SET_OBJECT(to, isObject(from));
   }
   return to;
 }