This is a first attempt at exposing a regression in Grape 1.3.1 compared
with 1.2.4 with respect to specifying subclasses of Grape::Entity as
parameters.

The addition of a GrapeEntityCoercer to handle subclasses of
Grape::Entity enables grape entity parameters to be specified without
resulting in the coercing logic to mark them as invalid.

A grape entity can expose an attribute with a different key. So we need
to handle that case as well.

Validation of grape entities now handles an array. However, we still
need to ensure that the coerced values are returned.