[THINGS THAT WILL BREAK YOUR CODE]
* Header handling has changed. There is no more package variable
%headers that holds all the headers to be added. They are now
added on a per-object basis.
If you were adding a header with add_header(), and the code
relied on that header still being set later on in a later
instance of the variable, that code will now break, because
the later instance won't have the header set.
* You can now prevent a header from being sent by adding it with
an undef value, as in:
$mech->add_header( Referer => undef );
* Now correctly adds Accept-Encoding to all requests that need it.
* Added new $mech->_modify_request($req) method to do all the
HTTP header modification before the actual request gets
sent off. Subclasses are able to override it if they want.
* Removed the unused Compress::Zlib stuff.