Fix the type inference error we keep getting in 2.10.
Revert "No need for an abstract executor as long as there's an implicit one."

This reverts commit d76a487c3bea8686c8828a85026c9a486de14e26.  It causes
problems in Akka 2.1, where the ExecutionContext is a parameter to each
operation.  Threading it through to renderResponse would have a huge
ripple effect on the API, including user extensions.
Collect works again.