Make request data extendable.

If for instance, this class is wrapped around a HTTP client,
or to extract data from logs.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5693 2fea6a4d-a838-0410-917b-93a53c48e9d2
Private properties to protected.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5694 2fea6a4d-a838-0410-917b-93a53c48e9d2
Allow passing the raw request data to constructor.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5695 2fea6a4d-a838-0410-917b-93a53c48e9d2
Documentation.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5696 2fea6a4d-a838-0410-917b-93a53c48e9d2