quick handling of default image values

[KeynesYouDigIt]

Closes #405

Why is this the best possible solution? Were any other approaches considered?

This is the simplest and fastest way to get default support for images. I am creating another pull request now which creates broader support for different binary defaults.

What are the regression risks?

image questions now have another processing step. Nothing in this step appears unsafe, but if there is a regression, the new step would be the first place to look. Given this is a small change, in the unlikely event of a regression, I believe the fix will be easy.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

I do not believe so.

Before submitting this PR, please make sure you have:

• [x] included test cases for core behavior and edge cases in tests_v1
• [x] run nosetests and verified all tests pass
• [x] run black pyxform to format code
• [x] verified that any code or assets from external sources are properly credited in comments

[KeynesYouDigIt]

Can I merge this or does someone else have to? I don't see the button!

[KeynesYouDigIt]

Thanks so much @ukanga !!!! ping me right away if you encounter any issues!!!

Edit - Looks like Black is failing for some reason?? I thought I ran it locally, plus I thought CI ran it when the PR was opened!! I will fix this right away when I get home and open another PR 🤦

Edit 2 - link to failing build https://app.circleci.com/pipelines/github/XLSForm/pyxform/899/workflows/7e316fff-4a92-46ac-8a3d-c23b56cbfde3/jobs/3048/steps

[KeynesYouDigIt]

@ukanga / @lognaturel it looks like AppVeyor is also failing in CI because of an intermitent networking issue. Not sure who needs to know or can fix/rerun to try again.

https://ci.appveyor.com/project/xlsform/pyxform/builds/41136312/job/k22b98wmp73k13cx

[yanokwa]

Fixed at https://github.com/XLSForm/pyxform/pull/565.

[lognaturel]

Thanks, all!