Fix tab completion appending quote

[tompng]

Fixes #444

When complecion candidates are %w[foo bar baz]

input before tab complete	master	this pull request	readline-ext
x = "f█	x = "f"█	x = "foo"█	same
x = 'f█	x = 'f'█	x = 'foo'█	same
x = "b█	x = "b"█	x = "ba█	same
["a", "f█	["a", "f"█	["a", "foo"█	same
["a"b", "f█	["a"b", "f█	["a"b", "foo█	["a"b", "foo"█
["a\"b", "f█	["a\"b", "f"█	["a\"b", "foo"█	same
x = "f█ bar baz	x = "f"█ bar baz	x = "foo█ bar baz	same

require 'readline'
Readline.completion_proc = proc { |word,preposing|
  %w[foo bar baz].select { |w| w.start_with?(word) }
}
Readline.readline('> ')

[ima1zumi]

LGTM!

[st0012]

Do we ever use its argument?

[tompng]

I left the parameter because this is exposed to DialogProcScope like this.

# This is possible
Reline.add_dialog_proc(:dialogname, ->{
  p retrieve_completion_block(true)
  nil
})

But code searching in github, it looks like this method is not used yet. I think we can remove the parameter.

I didn't confirm yet, but this method itself might be useful to use from IRB to reduce this hack
https://github.com/ruby/irb/blob/84366b8cdd7ac4c200a03405a07ed074aff5ae23/lib/irb/input-method.rb#L302-L305
Hack that stores @completion_params in completion_proc and read it from dialog_proc