Fixed the fact that when trying to update a custom field in an issue a PHP error gets generated.

Previously, the code would loop through all key-value pairs and try to attach the value as a string.

When hitting key='custom_fields' it would encounter an array as the value and this would fail.

There was existing additional logic to deal with this scenario when creating a new issue, but
not when updating an old one.

I have moved the code from create() that generates the XML into into its own function, and then
called this function from create() and update(). This seems to fix the issue.
Merge pull request #20 from timporter/master

Warning: SimpleXMLElement::addChild() expects parameter 2 to be string, array given in Redmine/Api/Issue.php on line 145