Fix default and additional key bindings vanish bug

[tompng]

When inpurc file exists, default key bindings and additional key bindings were cleared after first Reline.readline.

irb(main):001> 123 (arrow key works)
=> 123
irb(main):002> 123 (arrow key does not work)

Changes

Config#read

Changed config.read always turns @loaded to true. This will make the behavior difference of inputrc file existence small.
Even if inputrc file does not exist, we can still think null file is loaded. This will also avoid attempt loading invalid inputrc file many times.

Config#reset

Config is loaded only once and sets additional_key_bindings. We can't clear additional_key_bindings. We don't need to clear default_key_bindings too.

[ima1zumi]

Thanks for finding and fixing this bug. 🙏