Thanks for the bug report. I will try and fix it in the next update.
The speedy library is actually a standalone library developed by
http://aciddrop.com/
The Plugin provides integration of the library with Joomla. So the fixes have to be made into the standalone library and approved by the developer before getting into the Joomla plugin. So it takes a bit more time.
I've decided to branch the PHP Speedy library in Joomla so updates can be quicker. The source is availabe via subversion at:
https://joomla-php-speedy.googlecode.com/svn/trunk/
Project page:
http://code.google.com/p/joomla-php-speedy/
If you need update access let me know.
The file you probably want to look at is:
http://code.google.com/p/joomla-php-speedy/source/browse/trunk/plug_speedy-J1.5/php_speedy/controller/compressor.php