[BUG] Nested builder parentage don't work

This kind of structure (though where each builder is in an `#export` method on a model of some sort, not quite this simple) is very useful for composing a document, but it currently fails like so:

```
  1) Error:
test_builder_inside_builder(Nokogiri::XML::TestBuilder):
NoMethodError: undefined method `parent=' for nil:NilClass
    /Users/ben/code/nokogiri/lib/nokogiri/xml/builder.rb:385:in `insert'
    /Users/ben/code/nokogiri/lib/nokogiri/xml/builder.rb:377:in `method_missing'
    /Users/ben/code/nokogiri/test/xml/test_builder.rb:303:in `block (3 levels) in test_builder_inside_builder'
    /Users/ben/code/nokogiri/lib/nokogiri/xml/builder.rb:293:in `initialize'
    /Users/ben/code/nokogiri/lib/nokogiri/xml/builder.rb:254:in `new'
    /Users/ben/code/nokogiri/lib/nokogiri/xml/builder.rb:254:in `with'
    /Users/ben/code/nokogiri/test/xml/test_builder.rb:302:in `block (2 levels) in test_builder_inside_builder'
    /Users/ben/code/nokogiri/lib/nokogiri/xml/builder.rb:393:in `call'
    /Users/ben/code/nokogiri/lib/nokogiri/xml/builder.rb:393:in `insert'
    /Users/ben/code/nokogiri/lib/nokogiri/xml/builder.rb:377:in `method_missing'
    /Users/ben/code/nokogiri/test/xml/test_builder.rb:301:in `block in test_builder_inside_builder'
    /Users/ben/code/nokogiri/lib/nokogiri/xml/builder.rb:293:in `initialize'
    /Users/ben/code/nokogiri/test/xml/test_builder.rb:300:in `new'
    /Users/ben/code/nokogiri/test/xml/test_builder.rb:300:in `test_builder_inside_builder'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:942:in `run'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:781:in `block in _run_suite'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:774:in `map'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:774:in `_run_suite'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:764:in `block in _run_suites'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:764:in `map'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:764:in `_run_suites'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:740:in `_run_anything'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:903:in `run_tests'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:890:in `block in _run'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:889:in `each'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:889:in `_run'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:878:in `run'
    /Users/ben/.rvm/gems/ruby-2.0.0-p195/gems/minitest-2.2.2/lib/minitest/unit.rb:658:in `block in autorun'
```

Any help in figuring out why this is would be appreciated.