Add a fallback for Array methods

For the sake of backward compatibility, we need to provide a fallback
for missing method that may be used in the existing libraries.

We also need to redefine #== as checks between PartsList objects and
arrays were valid before.

Also define the PartsList#respond_to? method since we are relying on a
method_missing hack to bring Array methods to the PartsList object, we
need to override #respond_to? to make sure that any existing check will
not fail.

Moreover, since we are not relying on Array's encode_with method, the
YAML loading from a serialized Mail::PartsList object doesn't return an
Array anymore.
Let's remove Rubinius from the allowed failures