de-complect ring response generation from representation

This change allows implementations of the Representation protocol to act on values nested inside data structures.

So, for example, if I have `defrecord Foo` in my code and extend the Representation protocol for Foo's, that will work if my handlers return one Foo record, but it would fail if a handler returned a vector of Foo's (because the render-seq-generic would hand off that vector to the output encoder, which wouldn't know how to handle Foo records).

We're unable to get the test suite to run locally, so we haven't written any tests around this (or updated the existing tests). If someone can help us figure out the problem there (I posted to the Google Group about it), then we'd be happy to add test coverage and update the existing tests.