move class DokuHTTPClient behind HTTPClient

2 different people reported errors that seems to be solved when following the "If a class extends another, then the parent class must be declared before the child class structure." of http://php.net/manual/en/language.oop5.inheritance.php
Couldn't exactly reproduce under which php/environment the problem occurs, but following the MUST of the PHP documentation seems not to be wrong..

https://github.com/Flyspray/flyspray/pull/647