Expose url_path_for on HTTPConnection

[judahrand]

Summary

Based on the conclusion of #604 which can be found here the consensus seems to be that trying to munge host headers when located behind a reverse proxy isn't the best approach and that instead absolute paths should be used in redirects. The same, I imagine, should apply in tempates.

This is currently possible but would be made easier/neater if the url_path_for method which exists on Starlette was exposed on the HTTPConnection and therefore the Request. Additionally, adding a url_path_for jinja function would allow for this to be achieved in templates much easier.

I leave this PR here for review with the knowledge that if this approach is approved of docs will also need to be added.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[alex-oleshkevich]

self.url_for returns and instance of URL that you can use in routes/templates to get a path: self.url_for(name).path (you have it already done in the PR).
I don't think we need another method.

The difference between Starlette.url_path_for and HTTPConnection.url_for is that Starlette app can be used in non-HTTP contexts (like CLI or background tasks) and therefore has no Host concept. In all other cases I advise using url_for.