Fix srcset support

[Cardinal90]

Justification is in commit comments

[alexanderdickson]

Hi @Cardinal90! Thanks for the PR.

Some things

• You test against undefined, I guess because they can return empty strings?
• Can you add relevant tests? There is a qunit file.
• Can you bump the version to 2.5? It should be automatically published to npm now with a recent change

Thanks!

[Cardinal90]

Hi, sorry for the long delay.

• That check is done against new Image() with no attributes. If srcset is supported, it is an empty string, else undefined.
• I don't see a good way to add tests in this case. You'd have to assert that srcset and sizes attributes are properly copied onto an internal image instance. From the perspective of library's user (or the test file) the callback is called either way. With src attribute of about: all images in your tests error out anyway, but even using real images I don't see how to test this.
• Bumped version.