Bug fix for issue 1035

[tim-reynolds]

#1035

A bug in the appendChild method of Magento\Framework\Simplexml\Element.php causes values to be lost in very specific cases. This will fix it. The existing unit test for appendChild has been updated as well.

Bug: When appending a node that has an attribute and no child nodes, the code will take the code path for having children. This is because the result of $source->children() evaluates to TRUE when a node has attributes and no children.
Example:

$root = simplexml_load_string('<root/>', 'Magento\Framework\Simplexml\Element');
$append = simplexml_load_string('<child attr="abc">Text</child>', 'Magento\Framework\Simplexml\Element');
$root->appendChild($append);
echo $root->asNiceXml();

Should output:

<root>
    <child attr="abc">Text</child>
</root>

Actually outputs:

<root>
    <child attr="abc" />
</root>

Fix: Changing to call from [children()] to [count()] fixes the issue.

[anupdugar]

Thanks for submitting this pull request. We will process it internally through ticket MAGETWO-34069 and merge your pull request after that.