string.c: use str_duplicate

* string.c (rb_str_resurrect): use str_duplicate(), which does
  completely same.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -1006,7 +1006,7 @@ rb_str_resurrect(VALUE str)
 	RUBY_DTRACE_STRING_CREATE(RSTRING_LEN(str),
 				  rb_sourcefile(), rb_sourceline());
     }
-    return str_replace(str_alloc(rb_cString), str);
+    return str_duplicate(rb_cString, str);
 }
 
 /*