Fix one JUnit formatter scenario (WIP)

The reason this one fails is that the stack trace in the failing scenario isn't quite right. But that was obscured because our new DocString wrapper class doesn't readily compare to a String when used in an RSpec assertion.

There are two things to do here:

- [ ] change `Cucumber::MulitlineArgument::DocString` so it reports better assertion failure messages without having to be converted to a string first.
- [ ] fix the stack trace in the error reported to the formatter.