Add User-Agent header to XML-RPC requests

As of last week, an undocumented change was made to WordPress.com blogs which prevented XML-RPC requests being accepted unless the User-Agent header was present (see discussion [here](http://en.forums.wordpress.com/topic/xml-rpc-failing-to-respond-to-post-requests-via-curl-in-php?replies=7#post-2003632)).

The [XML-RPC specification](http://xmlrpc.scripting.com/spec.html) itself does state that the User-Agent header is required. This PR adds a meaningful User-Agent header to both cURL and file requests.