As described in #481, adding a protected field like `legal_entity` as
part of an update API operation can cause some issues like a custom
encoding scheme not being considered and special handling around empty
values being ignored.
As a an easy fix for this, let's disallow access to protected fields in
the same way that we disallow them from being set directly on an
instance of a given model.
Helps address (but is not a complete fix for) #481.
* Rename the `Update` operation to `Save`
* Add the `update` class method to all saveable resources
* Add tests for update method
* Add tests for plans, invoice items, and application fees
