Scope generated route helpers to an instance of RoutesProxy

[f-mer]

Motivation / Background

This Pull Request has been created because RoutesProxy caches route helpers within its class using self.class.class_eval(...). For example if two engine Weblog::Engine and Metrics::Engine are mounted within your app and only weblog defines the route helper weblogs_path calling metrics.respond_to?(:weblogs_path) will return true if previously weblog.weblogs_path has been called.

Detail

This Pull Request changes self.class.class_eval(...) to instance_eval(...) so route helpers are cached within the instanced object instead of the class so they are not shared by all instances.

Additional information

I would love to get feedback if there is a better way to test this.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.