Add failing tests for fingerprinting srcset

[samdemaeyer]

Fingerprinting srcset is not working.

Expected:

<img src="https://cloudfront.net/images/my-folder/my-image-fingerprinted.png" srcset="https://cloudfront.net/images/my-folder/my-image-fingerprinted.png 1x, https://cloudfront.net/images/my-folder/my-image-fingerprinted_@2.png 2x" />

Actual:

<img src="https://cloudfront.net/images/my-folder/my-image-fingerprinted.png" srcset="https://cloudfront.net/images/my-folder/my-image-fingerprinted.png 1x, /images/my-folder/my-image_@2x.png 2x" />

[samdemaeyer]

Issue #51

[rickharrison]

Thanks for the failing test!