Fix default rendered format problem when calling render method without :content_type option. Closes #11393.
Merge pull request #11437 from kennyj/fix_11393

Fix #11393. Fix default rendered format when calling render method without :content_type option.
re-introduce `select_for_count` private method.

See https://github.com/rails/rails/commit/da9b5d4a8435b744fcf278fffd6d7f1e36d4a4f2#commitcomment-3630064 for discussion.