Changed interceptor to append a Mail::Field instead of a Mail::DkimField...

Resolving issue https://github.com/jhawthorn/dkim/issues/8

Registered DkimField with Mail so field header class is not an OptionField.

This does fail test `Dkim::InterceptorTest#test_same_output_as_direct_usage` with Mail 2.5.3 as field order works differently.