Sometime between 5.3 and 5.6, PHP changed from considering the default
output buffer (ob_start() or ob_start( null )) as "user" to considering
it as "internal", which prevents wfResetOutputBuffers() from removing
What we really should do here is test directly for whether the buffer
can be deleted, using the 'del' flag in PHP 5.3 or 'flags' in PHP 5.4+.
As for HHVM, we'll need to continue falling back to testing 'type' for
now thanks to https://github.com/facebook/hhvm/issues/5563.