Better implementation of https://github.com/rails/rails/pull/41313

Now, only `false` and `:lower` as args will return a downcased first character (before https://github.com/rails/rails/pull/41313

 only `false` did). Everything else returns an upcased first character, which is the default.

Update activesupport/lib/active_support/inflector/methods.rb

Co-authored-by: Ryuta Kamizono <kamipo@gmail.com>