Make #const_missing consistent with MRI

Hello,

This pull request simply makes the `#const_missing` messages more consistent with MRI. I don't know if the commit are organized as you want ; if you want me to make two separate commits for the specs (one for the assertion on the message when the constant is really missing and one for the addition of the spec for private constants), let me know!

For the record, this disparity has been discovered fixing [this test](https://github.com/rails/rails/blob/a235105a5b38c793957ad065cc982b03d3d4ef51/activesupport/test/constantize_test_cases.rb#L75) in Rails reaching [this line](https://github.com/rails/rails/blob/df8b021102d96e5ccadc9a633e846bb33c8466c6/activesupport/lib/active_support/inflector/methods.rb#L306).

Have a nice day.