string.c: use rb_id_encoding

* string.c (rb_str_init): rb_id_encoding() returns same ID with
  caching.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52982 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -1340,7 +1340,7 @@ rb_str_init(int argc, VALUE *argv, VALUE str)
     int n;
 
     if (!keyword_ids[0])
-	keyword_ids[0] = rb_intern("encoding");
+	keyword_ids[0] = rb_id_encoding();
 
     n = rb_scan_args(argc, argv, "01:", &orig, &opt);
     if (argc > 0 && n == 1)