* Improve parsing of srcset according to whatwg spec; e.g. srcset="http://example.com/size400,300/img.jpg 640w"
* Trim/normalise the output in order to conform to prior version; solely to keep tests happy
* Add test case for embedded commas in a srcset url as well as support for future possible parenthesis in descriptor string
* Drop 'future proof' test as it causes an error message in test output