Closes #155, introduces #159, closes #151 with authority - inner `img` is now being evaluated for `srcset`/`sizes` in the event that no sources match.
Prevents `candidates` from being returned as not-an-array.
This should probably be a private method.
Might as well just pass the (potentially null) sizes value here, per https://github.com/scottjehl/picturefill/pull/160/files#r11493692
Merge pull request #160 from scottjehl/eval-img

Closes #155, closes #151, ref #159