Make guice-servlet more adaptable

_From [mcculls](https://code.google.com/u/108081409801028235828/) on March 22, 2011 13:51:37_

We're using guice-servlet in a multi-injector scenario where we want to chain the different filter/servlet pipelines under a single instance of a subclass of GuiceFilter. To get this working without co-locating our custom filter under the same package as GuiceFilter we need to make a couple of visibility changes (including removing final from an injected field).

The attached patch contains the necessary changes, comments welcome :)

**Attachment:** [gist](https://gist.github.com/3ee24f4bbe09cccaaf97)
   _[GUICE_SERVLET_CODE_VISIBILITY.patch](https://gist.githubusercontent.com/gissuebot/3ee24f4bbe09cccaaf97/raw/a3dc264a71e97b9220f8be3e3daedbd444965042/GUICE_SERVLET_CODE_VISIBILITY.patch)_

_Original issue: http://code.google.com/p/google-guice/issues/detail?id=618_