Add Client.VolumeInUse to AWS delete_volume mock

delete_volume when a volume is attached to a server should fail with
an exception.

lib/fog/aws/requests/ec2/delete_volume.rb

@@ -29,6 +29,10 @@ module Fog
         def delete_volume(volume_id)
           response = Excon::Response.new
           if volume = @data[:volumes][volume_id]
+            if volume["attachmentSet"].any?
+              attach = volume["attachmentSet"].first
+              raise Fog::AWS::EC2::Error.new("Client.VolumeInUse => Volume #{volume_id} is currently attached to #{attach["instanceId"]}")
+            end
             @data[:deleted_at][volume_id] = Time.now
             volume['status'] = 'deleting'
             response.status = 200

spec/aws/models/ec2/volume_spec.rb

@@ -43,6 +43,14 @@ describe 'Fog::AWS::EC2::Volume' do
       volume.destroy.should be_true
     end
 
+    it 'should fail if the volume is attached to an instance' do
+      server = AWS[:ec2].servers.create(:image_id => GENTOO_AMI)
+      server.wait_for { ready? }
+      volume = AWS[:ec2].volumes.create(:availability_zone => server.availability_zone, :size => 1, :device => '/dev/sdz1')
+      volume.server = server
+      lambda { volume.destroy }.should raise_error
+    end
+
   end
 
   describe "#server=" do