add test for how we handle compressed files

this is insufficient coverage, there are other methods that likely
support decompression. this test is only intended to demonstrate an
instance of zlib/lzma support, relative to libxml2's "legacy" option
in v2.13.0.

ext/nokogiri/nokogiri.c

@@ -203,6 +203,9 @@ Init_nokogiri(void)
   rb_const_set(mNokogiri, rb_intern("LIBXSLT_COMPILED_VERSION"), NOKOGIRI_STR_NEW2(LIBXSLT_DOTTED_VERSION));
   rb_const_set(mNokogiri, rb_intern("LIBXSLT_LOADED_VERSION"), NOKOGIRI_STR_NEW2(xsltEngineVersion));
 
+  rb_const_set(mNokogiri, rb_intern("LIBXML_ZLIB_ENABLED"),
+               xmlHasFeature(XML_WITH_ZLIB) == 1 ? Qtrue : Qfalse);
+
 #ifdef NOKOGIRI_PACKAGED_LIBRARIES
   rb_const_set(mNokogiri, rb_intern("PACKAGED_LIBRARIES"), Qtrue);
 #  ifdef NOKOGIRI_PRECOMPILED_LIBRARIES

test/xml/sax/test_parser.rb

@@ -274,6 +274,16 @@ def call_parse_io_with_encoding(encoding)
           end
         end
 
+        it "parses a compressed file" do
+          skip("libxml2 legacy support") unless Nokogiri.uses_libxml? && Nokogiri::LIBXML_ZLIB_ENABLED
+
+          filename = XML_FILE + ".gz"
+          parser.parse_file(filename)
+
+          refute_nil(parser.document.start_elements)
+          assert_operator(parser.document.start_elements.count, :>, 30)
+        end
+
         it :test_render_parse_nil_param do
           assert_raises(TypeError) { parser.parse_memory(nil) }
         end