Cannot unmarshal into nameless class

Currently this library relies on class names in order to instantiate objects while unmarshaling, even if you have explicitly registered a tag via add_tag. Consequently, nameless classes do not properly survive a round trip.

require 'yaml'

w = Class.new do
    def initialize(foo)
        @foo = foo
    end
    def foo
        @foo
    end
    def encode_with(c)
        c['bar'] = @foo
    end
    def init_with(c)
        initialize(c['bar'])
        self
    end
end
YAML.add_tag('!sample', w)

x = w.new('abc')
y = x.to_yaml # "--- !sample\nbar: abc\n"
z = YAML.load(y) # {"bar"=>"abc"}
z.foo # NoMethodError (undefined method `foo' for {"bar"=>"abc"}:Hash)

Start with the YAML.add_tag and YAML.load paths shown in the reproduction, then trace how the registered tag is resolved when the class has no name. Re-run the provided example as a regression case; done means loading the YAML returns the registered object and z.foo returns "abc".