Fixed: warning: class variable access from toplevel on Ruby 2.0 (#14511).

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12052 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/SVG/Graph/Graph.rb

@@ -1,8 +1,7 @@
 begin
   require 'zlib'
-  @@__have_zlib = true
 rescue
-  @@__have_zlib = false
+  # Zlib not available
 end
 
 require 'rexml/document'
@@ -211,7 +210,7 @@ module SVG
         @doc.write( data, 0 )
 
         if @config[:compress]
-          if @@__have_zlib
+          if Object.const_defined?(:Zlib)
             inp, out = IO.pipe
             gz = Zlib::GzipWriter.new( out )
             gz.write data