Fix Channel#unsubscribe

Hi there!
Here is a simple fox for unsubscribbing from channel. I spent some time for figuring out why this code does not work:
```ruby
module Herald
  class ChartController < FayeRails::Controller
    channel '/chart' do
      subscribe do
        # ...
      end
    end

    def self.before_remove_const
      @channels.each(&:unsubscribe)
    end
  end
end
```
Rails called before_remove_const method on every class reloading but it had no effect. On every class loading subscriptions count just increased.
As it turned out Faye::Protocol::Channel#unsubscribe expects string but receives Faye::Subscription object =(
And no exception =(
I fix a couple of lines and now all work fine.
But.
I cannot write a test for it because there is one already. And it does not work. Here is the code:
```ruby
it "should not receive messages after unsubscribe" do
  Fiber.new do
    this_fiber = Fiber.current
    controller.channel('/widgets/99') do
      subscribe do
        this_fiber.resume message
      end
      unsubscribe
    end
    EM.schedule do
      FayeRails.client.publish('/widgets/99', "Welcome to Spacely Sprockets.")
    end
    EM.add_timer 1 do
      this_fiber.resume 'timeout'
    end
    Fiber.yield.should == 'timeout'
  end.resume
end
```
First of all EM is supposed to run in a separate thread. And this is completely not compatible with Fiber#resume. You cannot call Fiber#resume from other thread.
Second, spec helper:
```code
Thread.new do
  run Dummy::Application
end
```
If you'll try to join this thread then you will see
```
faye-rails/spec/spec_helper.rb:16:in `block in <top (required)>': undefined method `run' for main:Object (NoMethodError)
```
So this is does not work either.
Third. 
```ruby
it "should not receive messages after unsubscribe" do
  Fiber.new do
    # ...
    Fiber.yield.should == '...'
  end.resume
end
```
Fiber execution interrupts on Fiber.yield call. And #should method is never called.
So controller_spec.rb must be massively rewritten because it does not test anything. I can do something about it if it will be welcome.