When you say "wrapper function around headers_list()", that would also imply to have a wrapper around send_header(), as it is currently the case in official version of `FirePHP_Test_Class`, right ? And those wrappers both works with cached headers (but in unit tests version only). If this is correct, then I don't think we can accurately test that "normal" headers sent by an application are also taken into account, because this would require to use the wrapped version of send_header() and thus test the fake `FirePHP_Test_Class` and not actual code.

TL;DR: I feel tested code and behavior diverge too much from actual code to be relevant. Is this tolerable ?