Fix special methods accessibility.

[ksss]

I have fixed the problem regarding visibility of special methods.

Background

Special methods are default private instance methods in ruby.

• #initialize
• #initialize_copy
• #initialize_dup
• #initialize_clone
• #respond_to_missing?

Problem

String#initialize_copy is public method in RBS

$ bundle exec rbs method String initialize_copy
::String#initialize_copy
  defined_in: ::String
  implementation: ::String
  accessibility: public

String#initialize_copy expected to be private method.

public def initialize_copy is private method in RBS

class Foo
  public def initialize_copy: (self) -> self
end

$ bundle exec rbs method Foo initialize_copy
::Foo#initialize_copy
  defined_in: ::Foo
  implementation: ::Foo
  accessibility: private

If it has a public modifier, it expected to be a public method.