Add FileUtils copy_file method.

Was getting failures with rack v1.4.5 (same issues exist from 1.3.0 to 1.4.5).

````
     Failure/Error: Rack::Utils::Multipart::UploadedFile.new(file_path, "text/plain")
     NoMethodError:
       private method `copy_file' called for FakeFS::FileUtils:Module
````

https://github.com/rack/rack/blob/master/lib/rack/multipart/uploaded_file.rb#L17

FileUtils.copy_file is defined as a public method (at least in Ruby 1.8.7).  

To fix this, I added the copy_file method (which accepts 2 additional params besides src and dest.)
See:
https://github.com/ruby/ruby/blob/ruby_1_8_7/lib/fileutils.rb#L462

The two additional params will just get ignored, but it looks like FileUtils cp definition only takes 2 args instead a third (options) arg, so I didn't think that'd be an issue.