Validate path variables. Fixes #865

[aldenquimby]

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• Have you followed the guidelines in our Contributing document?
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• Have you written unit tests?
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Closing issues
Closes #865

If this is a new feature submission:

• Has the issue had a maintainer respond to the issue and clarify that the feature is something that aligns with the goals and philosophy of the project?

Potential Problems With The Approach

• Breaking change. Would any users rely on { or } being part of their path?

Test plan

• Unit tests show {param} -> :param replacement still works, and that errant { or } now throw

[WoH]

Should route/swagger behave differently? Obviously not. However, I'd say that's for now mainly a bug with the square bracket conversation.
I'm don't know what each of the possibly underlying frameworks do with square brackets, I'd hope they auto escape/accept the raw and the %endoded form.

The current implementation is already too aggressively replacing things, so I'd rather not do more of that.

[aldenquimby]

@WoH good point. I updated the implementation to match swagger. So instead of throwing if there are floating { or } in a path, route generation just won't replace them.

Note - I also renamed from brackets [] to braces {} because I was using the wrong term

[WoH]

LGTM, thanks a lot.