Add possibility to set attributes of the nodes with text in SimpleXML

Test case:
```php
<?php
$xml = <<<XML
<test>
  <tag1 />
  <tag2>content</tag2>
</test>
XML;

$test = simplexml_load_string($xml);

$test->tag1['foo'] = 'foo';
echo $test->tag1['foo'] . PHP_EOL;

$test->tag2['bar'] = 'bar';
echo $test->tag2['bar'] . PHP_EOL;

echo $test->asXML();
```

PHP output:
```
foo
bar
<?xml version="1.0"?>
<test>
  <tag1 foo="foo"/>
  <tag2 bar="bar">content</tag2>
</test>
```

HHVM output before:
```
foo
HipHop Fatal error: cannot create attribute on this node in test.php on line 14
```

HHVM output after:
```
foo
bar
<?xml version="1.0"?>
<test>
  <tag1 foo="foo"/>
  <tag2 bar="bar">content</tag2>
</test>
```

It is used in Symfony 2 unit tests.