Checking in the new PassThroughFilter class

This checkin adds the new PassThroughFilter class. This is a very simple class: it just pass along raw bytes without modification. However,
despite its simplicity, it can do a number of things for us:

- It works as a great sample of how to write a ContentFilter, and also one that deals with byte ranges.
- It can be used to debug a stream of bytes, because it can be put in the middle of a filter chain and you can inspect bytes in it.
- It is quite easy to modify, therefore it can be used to write logs about, for example, how often a given resource is read.
- It can be used to debug the ContentFilter mechanism itself.

As I said, feel free to copy and modify this class to do whatever you may need.