This fixes several things:
- root nodes without children have to return false.
- some_node->children() has to have children to be true.
- child nodes without children are always true!

This already happens using simplexml_load_string, but it must also
happen when directly instantiating and specifying the XML through
constructor.

Perhaps this is not the best implementation. But at least it will allow
us to access cdata.

Most public attributes can be literally copied to the cloned node.
For attributes and children, they have to be regenerated.

- When using specifying a value to addChild, SimpleXML does not display
it when the node is casted to a string, see the following test:
  hphp/test/slow/simple_xml/1641.php

- However, when you cast the return value of this call to string, it
does show a return value, see the following test:
  hphp/test/slow/simple_xml/1656.php (line 13)

- Additionally, this fixes "string nesting" because m_children needed to
be merged with the newly created string element due to the way strings
work in this extension.

Another test was added combining all these conditions.

Just like properties, when dealing with integer based lookups, we need
to iterate ourselves and pick the appropriate child.

SimpleXML will implicitly convert string nodes to actual strings when
the parent node is casted to an array. It will only do this for top
level nodes. A test has been provided.

Once you cast a root element to an array, PHP will consider top level
child nodes as new 'root nodes', these nodes behave differently to child
nodes when it comes to boolean casting.

xmlIsBlankNode has to be called regardless of being a child or parent
text node.

This allows the user to access a node's attribute through
attributes()->some_attribute.

This does not fix the fact that SimpleXML considers attributes to be
actual SimpleXML nodes, rather than just strings. In practice this
should not matter. Such a fix would deserve its own commit.