I would much prefer not to introduce a dependency on XDebug for unit tests.

I think using `FirePHP_Test_Class` the way it is done now is sufficient.

We just need a wrapper function around headers_list().

One test should pre-populate some headers so that headers_list() will return some data that an application may have sent before FirePHP sends any data. That is also the reason why I want to use headers_list() all the time and not cache as the application may send headers as well.