Fixed an AttributeError triggered when Mapping.get_attribute_map was
called without passing a key.
Improved the behavior of the DataElementAttributeProxy. Alert: Breaking
change!

Will now raise an AttributeError when an attribute is accessed that is
not set on the underlying data element. This is important for PATCH
requests and needs to be distinguished from setting the value of an
attribute to None.
Merge branch 'master' of https://github.com/cenix/everest
Merge branch 'master' into v17x