REP-729 Add Header Filter

@dimtruck actually wrote the value as an attribute, but I changed it...

... for two reasons:
1. As you pointed out, it enable the usage of CDATA. Since header value restricted characters and  XML restricted characters are not an equivalent set, this helps avoid some corner cases.
2. The space character is used as a delimiter by a XML list. It is also a valid header character. So if we are allowing configuring of multiple values per header, we really shouldn't use a XML list. 

Of course, a solution to both of your comments could be to have the user configure header name/value pairs. Then adding multiple values could be achieved by defining multiple pairs with the same name. The only potential issue there is that the meaning of "overwrite" becomes more ambiguous. I would take it to mean remove any header that I've configured to overwrite, then add all of my configured headers. Using one of our header managers, that's probably the behavior we would see since changes are batched.