Allow attributes to be set to nil

[brianstien]

This adds logic to check a repeated string field called 'nullify'
on incoming proto messages.

For direct attributes, if there is an attribute with the same name
as an attribute specified in the nullify field, this attribute will be
set to nil.

For transformed attributes there is now an option when the transformer
is declared that will specify a name in the nullify field to inspect.
If the our key is in the nullify array of an incoming proto, the
transformer will set the transformed attribute to nil.

[liveh2o]

This is following the previous pattern, but I think it might cleaner to simply call this callable here and eliminate the else clause below. The code will be tighter and it's one less variable to initialize.

[liveh2o]

Or we can clean it up even more with something like this:

callable = args.first || block

if callable.is_a?(Symbol)
  callable = lambda { |value| self.__send__(callable, value) }
else
  raise AttributeTransformerError unless callable.respond_to?(:call)
end

It will skip the callable check when we know that we've just created something that is callable.

[liveh2o]

I like this pattern. It solves a very real problem with using Protocol Buffers and AR, and opens the door for future extensions and options.

Other than my inline comments, would you mind adding a section to the README explaining the problem and how this solves it?

[liveh2o]

👍