It turns out that the suckiness of Zuul with multipart requests
comes almost entirely from the Multipart handling in Spring's
DispatcherServlet. This change makes the proxy routes available
on an alternative path /zuul/<normal_path> (where
/zuul is the default value of zuul.servletPath). I have
tested those with 800MB file uploads using the main method in
the FormZuulServletProxyApplicationTests and the main
observation is that there is no OutOfMemory error (no-one tries
to download the complete request body). It works with Ribbon
and with the simple (HttpClient) filter. With Ribbon you
will need to set some timeouts if you want to upload files
as large as that, e.g. see application.yml in the tests:

hystrix.command.default.execution.isolation.thread.timeoutInMilliseconds: 60000
ribbon:
  ConnectTimeout: 3000
  ReadTimeout: 60000

You need to set "Transfer-Encoding: chunked" in the
incoming request. Chrome does not do this by default
apparently, but I was able to test with curl, e.g.

$ curl -v -H "Transfer-Encoding: chunked" \
  -F "file=@mylarg.iso" \
  localhost:9999/zuul/direct/file

The old proxy paths through the DispatcherServlet are still
available (for backwards compatibility and for convenience of
having the paths available at the root of the context path).

Fixes gh-254

Because of the way that a FormBodyServletRequestWrapper was
implemented (extending the Zuul servlet 2.5 wrapper) it could
barf at runtime if anyone called its servlet 3.0 methods. The fix
for that was to extract our Servlet30RequestWrapper and extend that
instead.

Also tweaked the DebugFilter a bit so it doesn't try and display
the whole payload. Probably speeds up file uploads a bit but the
fact that we have to store the whole request body in memory is
going to kill us eventually.

See gh-254