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.
11a6eec5f5