Set default value for Reline.input, Reline.output and Reline.special_prefixes

[ima1zumi]

Close #774

I have changed that default values are assigned to Reline.input=, Reline.output=, and Reline.special_prefixes= when nil is passed to these methods. This change aligns the behavior of Reline with that of readline-ext.

readline-ext sets special_prefixes=nil to nil, but I think the method name suggests that a string is expected. Reline.special_prefixes has been modified to default to an empty string instead of nil.

❯ ASDF_RUBY_VERSION=2.6.10 irb
irb(main):001> Readline.special_prefixes = nil
nil
irb(main):002> Readline.special_prefixes
nil

readline-ext

https://github.com/ruby/readline-ext/blob/9ee1b1a2f848ebb98b8ea07969a100dd24548411/ext/readline/readline.c#L561-L629

https://github.com/ruby/readline-ext/blob/9ee1b1a2f848ebb98b8ea07969a100dd24548411/ext/readline/readline.c#L1463-L1494

[tompng]

Reline::Windows have attr_writer :output but does not have input= method and it does not support changing input.

We need a check like before, or add a no-op input= method to Reline::Windows.

[tompng]

Maybe just val = STDOUT is enough and simple.