Fix writing to an IO with no external_encoding

A file-IO has no external_encoding by default, so that rb_to_encoding() fails with a TypeError.

This regression was introduces in commit 2e260f5.

Fixes #2752

ext/nokogiri/nokogiri.c

@@ -112,7 +112,8 @@ noko_io_write(void *io, char *c_buffer, int c_buffer_len)
 {
   VALUE rb_args[2], rb_n_bytes_written;
   VALUE rb_io = (VALUE)io;
-  rb_encoding *io_encoding = rb_to_encoding(rb_funcall(rb_io, id_external_encoding, 0));
+  VALUE rb_enc = rb_funcall(rb_io, id_external_encoding, 0);
+  rb_encoding *io_encoding = RB_NIL_P(rb_enc) ? rb_ascii8bit_encoding() : rb_to_encoding(rb_enc);
 
   rb_args[0] = rb_io;
   rb_args[1] = rb_enc_str_new(c_buffer, (long)c_buffer_len, io_encoding);

test/xml/test_node.rb

@@ -679,6 +679,14 @@ def test_write_to_with_block
           end
         end
 
+        def test_write_to_file_without_encoding
+          io = Tempfile.new
+          xml.write_to(io)
+          io.rewind
+          assert_equal(xml.to_xml, io.read)
+          io.close
+        end
+
         def test_serialize_with_block
           called = false
           conf = nil