fixing multiple images, keys method and adding row height config

[stewartsims]

Hi! Fixed a couple issues here:

• I noticed when using multiple images the first one was repeated: just a couple places where the index of the image was not being respected during iteration

• Added the addition from another PR for specifying row height e.g.

ws['!rows'] = [
    {hpx:275},
    {hpx:100},
    {hpx:100}
];

• I found the keys method threw an error in some situations when it was being passed a 'null' argument, just made it more defensive but possibly a symptom of a problem elsewhere in the code

Feel free to merge or not as you like, as we've discussed this project is not really being maintained but these changes along with your original ones might help someone else out there who needs this capability.

[mgreter]

Merged! Thanks a lot!

[JohnRSim]

Any chance you could get these updates merged with sheetjs repo 🥇
It looks like it's back to being maintained and worked on 👍 with the 0.93 release just coming out a day ago. - looks like there were some conflicts with the initial merge attempts