Fix warnings in test suite.

This resolves all the ruby errors that spit out when running the test suite. Minor, but annoying.

```
/Users/kylev/Work/nokogiri/test/html/test_document.rb:224: warning: ambiguous first argument; put parentheses or even spaces
/Users/kylev/Work/nokogiri/test/html/test_document.rb:239: warning: ambiguous first argument; put parentheses or even spaces
/Users/kylev/Work/nokogiri/test/html/test_document.rb:251: warning: ambiguous first argument; put parentheses or even spaces
/Users/kylev/Work/nokogiri/test/html/test_document.rb:252: warning: ambiguous first argument; put parentheses or even spaces
/Users/kylev/Work/nokogiri/test/html/test_document.rb:258: warning: ambiguous first argument; put parentheses or even spaces
/Users/kylev/Work/nokogiri/test/xml/test_builder.rb:17: warning: assigned but unused variable - builder
/Users/kylev/Work/nokogiri/test/xml/test_document.rb:57: warning: ambiguous first argument; put parentheses or even spaces
/Users/kylev/Work/nokogiri/test/xml/test_document.rb:379: warning: method redefined; discarding old test_prepend_child_fragment_with_multiple_nodes
/Users/kylev/Work/nokogiri/test/xml/test_document.rb:371: warning: previous definition of test_prepend_child_fragment_with_multiple_nodes was here
/Users/kylev/Work/nokogiri/test/xml/test_entity_reference.rb:29: warning: ambiguous first argument; put parentheses or even spaces
/Users/kylev/Work/nokogiri/test/xml/test_node.rb:861: warning: ambiguous first argument; put parentheses or even spaces
```

This feeble penance is offered in hope for assistance on #1138, which I haven't yet managed to debug and fix. :smiling_imp: