Add support for `HashWithIndifferentAccess#key?` and its aliases

This commit adds indifferent versions of `Hash#key?` and its
aliases (`Hash#has_key?`, `Hash#include?`, and `Hash#member?`) to
`Thor::CoreExt::HashWithIndifferentAccess`.

It works thus:

  hash = Thor::CoreExt::HashWithIndifferentAccess.new('foo' => 'bar')
  hash.key?('foo') # => true
  hash.key?(:foo)  # => true

Before this commit, `hash.key?(:foo)` would have returned `false`.