Revert "Add the current dispatcher to the context object for redispatch"

This reverts commit ddecadb44bd0da06ea5b22df2c65baccf17bd148.
Stash a reference to the current dispatcher in the psgi env

Forward needs access to the dispatcher for redispatch. First approach
was to stash it in the context object. That works, but looks a little
messy..

Instead stash the current dispatcher in the psgi env _before_ the
request object it created. A couple of lines of code shorter too!