Fix test_compression: actually set negative values before asserting

The negative-value loop was missing the assignment, so it asserted
against the stale value (9) left by the previous loop. Also remove
a Ruby 1.8 workaround comment.

Author: cfis

test/test_document.rb

@@ -43,12 +43,12 @@ def test_compression
       end
 
       10.upto(20) do |i|
-        # assert_equal(9, @doc.compression = i)
-        assert_equal(i, @doc.compression = i) # This works around a bug in Ruby 1.8
+        assert_equal(i, @doc.compression = i)
         assert_equal(9, @doc.compression)
       end
 
       -1.downto(-10) do |i|
+        @doc.compression = i
         assert_equal(0, @doc.compression)
       end
     end