Fix partial address update bug

[bhelx]

Resolves #309

It appears there are two problems biting us here:

1. The endpoint does not accept partial updates of addresses. You have to send the whole address each time.
2. address.changed_attributes.clear seems like it was mutating the reference to the attributes. That should probably be changed to be a copy of the attributes and not a reference.

Testing

You should see that when updating the account, every field is serialized for the Address.

require 'securerandom'

require 'logger'
Recurly.logger = Logger.new STDOUT

account = Recurly::Account.create(address: { city: "Brussels", country: "BE", zip: "1000" }, account_code: SecureRandom.uuid)
account.address.address1 = "1234 Rue du Lombard"
account.save!

[lmnleaf]

+1