Guice servlet doesn't map the default servlet /* properly

_From [limpbizkit](https://code.google.com/u/limpbizkit/) on May 03, 2009 16:39:30_

When you have a default servlet installed
  <servlet-mapping>
    <servlet-name>someservlet</servlet-name>
    <url-pattern>/*</url-pattern>
  </servlet-mapping>
...then Guice-servlet doesn't work. It doesn't recognize that /* is special. It returns "" as the servlet
path.

We should probably special case the empty servlet path, and treat it as if '/' was returned instead. http://java.sun.com/javaee/5/docs/api/javax/servlet/http/HttpServletRequest.html#getServletPath( )

_Original issue: http://code.google.com/p/google-guice/issues/detail?id=367_