URL Rewriter: 'Documentation on proxy' incorrect

Since this[0] commit, we shouldn't need the custom class anymore. The 'to' has a new parameter called drop-cookies, which by default seems to be 'false'. (I don't understand why though, the commit below seems to suggest the default is true).

We should update the XML Rule documentation[1] to add the drop-cookies parameter.

Question is though, since we just point to the upstream documentation[2] (which doesn't have this option), whether we should fork that as documentation as well? In which case we can ditch the whole proxy pass through section.

The only thing that puzzles me is that while the code seems to suggest 'true' is the default, in practice it seems to be 'false'. Which makes the 'proxy' type advanced rule also pass them.

Is this a bug or am I missing something? I.e. do we need to document how to disable the passing with cookies using an XML rule, or enable like currently?

[0] https://github.com/onehippo/urlrewritefilter/commit/615610fe9bc7195a7c77dc266f1e2ebe8d13e655

[1] https://onehippo-forge.github.io/url-rewriter/rules/xml-rule.html

[2] http://cdn.rawgit.com/paultuckey/urlrewritefilter/master/src/doc/manual/4.0/index.html