Image parser: sort options by their keys

This ensures deterministic output like

      style="border:1px solid black;margin:5px 5px 5px 5px"

instead of also allowing the alternative:

      style="margin:5px 5px 5px 5px;border:1px solid black"


This is in order to fix ImageTest unit test failures (when executed using JVM8): it relies on order-sensitive String comparison vs expected output.