fog/fog — pull request #1246 by masterzen (2012-11-03 15:31:04+00:00)
Title: [aws|compute] Fix #1245 - mock register_image fails with block device mapping

When calling register_image in mock mode with a block device mapping, it is failing with:

```
undefined method `[]=' for nil:NilClass (NoMethodError)
/Users/brice/devl/fog/lib/fog/aws/requests/compute/register_image.rb:108:in `register_image'
/Users/brice/devl/fog/lib/fog/aws/requests/compute/register_image.rb:107:in `each'
/Users/brice/devl/fog/lib/fog/aws/requests/compute/register_image.rb:107:in `register_image'
/Users/brice/devl/fog/lib/fog/aws/requests/compute/register_image.rb:100:in `each'
/Users/brice/devl/fog/lib/fog/aws/requests/compute/register_image.rb:100:in `register_image'
...
```

This changeset fixes the problem, and adds mock tests for register_image with block device mapping.