Rename MARC::Field to MARC::ControlField

MARC::Field does not exist.

/home/boutros/.rvm/gems/ruby-2.1.1/gems/marc-0.8.1/lib/marc/xmlwriter.rb:140:in `block in encode': uninitialized constant MARC::Field (NameError)

Maybe left after a renaming/refactoring?

I don't know, I just started to use this library.