Bugfix: render server HTTP response code bindings again (#1207)

In 171d76fc, we regressed on HTTP response code binding in (non-error)
operation outputs. We made the call to obtain the `Writable`, but we
never wrote it with our writer.

In that commit, I inadvertently added the `RestJsonHttpResponseCode`
test (which was previoulsy passing) to the list of failing tests without
noticing the regression.