Allow slashes in the final parameter in the path if it's name begins with a '+'

We've run into this with our own Discovery-enabled API, and while we could in theory deal with the unescaping server-side it'd be messy and we'd still have to handle mapping the parameter names correctly.

Per previous discussions, I think a new test under test/generated is the right thing to do - let me know if you'd rather I rolled it into an existing test.