More sugar for HashDef#[] and HashDef#[]=

[nicolasdespres]

Users should be able to write something like that:

# Check "HTTP_VERSION" is "HTTP/1.1".
h["HTTP_VERSION"] = "HTTP/1.1"
# Check "rack.version" is an array equal to [ 1, 1 ].
h["rack.version"] = [ 1, 1 ]
# Check "HTTP_USER_AGENT" matches /^Mozilla/.
h["HTTP_USER_AGENT"] =~ /^Mozilla/
# Check "rack.run_once" is boolean equal to false.
h["rack.run_once"] = false
# Check "action_dispatch.parameter_filter" is an array including :password.
h["action_dispatch.parameter_filter"].include_item :password
# Check "rack.session" is a hash containing a "session_id" key with a string value of minimum length 16.
h["rack.session"].has_property("session_id").string(min_length: 16)
# Check "rack.session" is a hash containing a _csrf_token" key with a string value of minimum length 44.
h["rack.session"].has_property("_csrf_token").string(min_length: 44)
# Check "rack.session.options" is a hash with the given hash schema.
h["rack.session.options"].object strict: false do |s|
  # A "secret" key with a string value of minimum lenght 60.
  s.string "secret", min_length: 60
  # A "httponly" key with a boolean value equal to true
  s.boolean "httponly", equal_to: true
  # A "rack.request.cookie_hash" hash with the following schema.
  s.headers "rack.request.cookie_hash" do |h|
    h["_blog_session"].string min_length: 218
  end
end
# Check "action_controller.instance" value is a ActionController::Base instance.
# This check won't be documented.
h["action_controller.instance"].is_a ActionController::Base, doc: false
# Check "__internal__" is a string equal to "foo" and won't be documented.
h["__internal__"].must_be("foo", doc: false)
# Check "REQUEST_URI" is string formatted as a URI.
h.uri "REQUEST_URI"

This would be useful for headers specification.