localeinit.c: fallback filesystem encoding to UTF-8

* localeinit.c (Init_enc_set_filesystem_encoding): fallback to
  UTF-8 on Mac OS X, just for convenience sake.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46202 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

localeinit.c

@@ -65,6 +65,9 @@ Init_enc_set_filesystem_encoding(void)
     if (idx < 0) idx = ENCINDEX_ASCII;
 #else
     idx = rb_enc_to_index(rb_default_external_encoding());
+# if defined __APPLE__
+    if (idx == rb_usascii_encindex()) idx = rb_utf8_encindex();
+# endif
 #endif
     return idx;
 }