RequestDispatcher do not follow the specification of Servlet 2.4

_From [gentaro.oka](https://code.google.com/u/gentaro.oka/) on February 11, 2012 22:45:29_

RequestDispatcher implementation in ManagedServletPipeline class and the specification of Servlet 2.4 do not accord.
 
Please see line 154 and ManagedServletPipeline.java https://code.google.com/p/google-guice/source/browse/extensions/servlet/src/com/google/inject/servlet/ManagedServletPipeline.java#154 "javax.servlet.forward.servlet_path" should be assigned the value of servlet path.
you can get it by calling HttpServletRequest#getServletPath().

When I executed GuiceServlet on Jetty server and used RequestDispatcher#forward(), I got ClassCastException and found this spec violation.
Because Jetty expect that "javax.servlet.forward.servlet_path" is String value.

_Original issue: http://code.google.com/p/google-guice/issues/detail?id=683_