Fixed using raw strings as requiredMsg

[grzesiek2010]

Closes #730

What has been done to verify that this works as intended?

I've tested the changes manually with ODK Collect and also added automated tests.

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

Nothing to discuss here.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

We can focus on testing custom retired messages. The fix is rather safe and isolated so I can't think of any risk in any other area of the project. However we should start testing once we implement changes in Collect and start using the new method for reading required messages.

Do we need any specific form for testing your changes? If so, please attach one.

I used the one attached in the issue.

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

No.