Add support for custom headers in the IXR (XML-RPC) class

[josephscott]

It's helpful to be able to add custom headers when sending an XML-RPC request from WordPress using the IXR class. I've included a patch to make this possible. While I'm there I've made a small change to make error codes more unique.

I've submitted these changes back to the IXR code:

​http://code.google.com/p/ixr/issues/detail?id=2

​http://code.google.com/p/ixr/issues/detail?id=3

[ryan]

(In [10313]) Add support for custom headers in the IXR (XML-RPC) class. Props josephscott. fixes #8145