A route with more then one variable in path, parameters in indeterminate order in output

With a route like

@app.route('/foos//bars/')

the output contains a path `/foos/{foo_id}/bars/{bar_id}`, but then
in `"get": { "parameters": [ ... ] }` the order of `foo_id` and `bar_id` is indeterminate between code reloads.

Function `rule_to_params` in `paths.py` accesses `rule.arguments` that is an instance of a set, and the order of arguments may therefore change. Maybe that is the reason?

swagger-codegen-cli produces client and server code that depend on the ordering of parameters.

Also, there doesn't seem to be a test case that has more then one variable in the path.

Start in paths.py at rule_to_params, where the issue identifies rule.arguments as a set, and review the existing path-parameter generation. Add coverage for a route with both foo_id and bar_id, then run the relevant test suite to verify the generated parameter order remains deterministic across reloads.

Written by the indexing model from the issue text.