forwardTo() behavior should be documented

[slightlyoff]

The fowardTo() API will generate redirects that need to have their behvaior spec'd.

In particular:

• behavior with regards to the generated request re-entering the controller
• how to handle loops
• determining what the eventual status code of the response will be

[jakearchibald]

Whatever forwardTo does, will redirects do the same? As in, if a page resource request isn't altered during onfetch, hits the network & redirects, does it reenter onfetch for the new url?

[annevk]

It seems cases like this is why you need integration with http://fetch.spec.whatwg.org/ as that already has a thin layer on top of HTTP (and sometimes overriding HTTP for bugs) and you can't pretend to just be a proxy.

[slightlyoff]

We decided that forwardTo() will re-enter, per decisions captured in #47. Redirects that are handed to the network layer are opaque and do not re-enter.