Enable setting any objects to :content! value

Setting any objects excluding String, Hash and Array to `:content!` value, and raise `NoMethodError` exception. Like this.

```ruby
Gyoku.xml(foo: {:'@bar' => 'a', :'@baz' => 'b', :content! => 10})
NoMethodError: undefined method `empty?' for 10:Fixnum
	from /Users/takayuki_atkwsk/sandbox/vendor/bundle/ruby/1.9.1/bundler/gems/gyoku-b429d6b2a6d7/lib/gyoku/hash.rb:55:in `block in iterate_with_xml'
	from /Users/takayuki_atkwsk/sandbox/vendor/bundle/ruby/1.9.1/bundler/gems/gyoku-b429d6b2a6d7/lib/gyoku/hash.rb:41:in `each'
	from /Users/takayuki_atkwsk/sandbox/vendor/bundle/ruby/1.9.1/bundler/gems/gyoku-b429d6b2a6d7/lib/gyoku/hash.rb:41:in `iterate_with_xml'
	from /Users/takayuki_atkwsk/sandbox/vendor/bundle/ruby/1.9.1/bundler/gems/gyoku-b429d6b2a6d7/lib/gyoku/hash.rb:12:in `to_xml'
	from /Users/takayuki_atkwsk/sandbox/vendor/bundle/ruby/1.9.1/bundler/gems/gyoku-b429d6b2a6d7/lib/gyoku.rb:13:in `xml'
	from (irb):2
	from /Users/takayuki_atkwsk/.rbenv/versions/1.9.3-p448/bin/irb:12:in `<main>'
```

This pullreq enables setting any objects to `:content!` value.

```ruby
Gyoku.xml(foo: {:'@bar' => 'a', :'@baz' => 'b', :content! => 10})
=> "<foo bar=\"a\" baz=\"b\">10</foo>"
Gyoku.xml(foo: {:'@bar' => 'a', :'@baz' => 'b', :content! => false})
=> "<foo bar=\"a\" baz=\"b\">false</foo>"
```