Added a new exception to detect the case where two

Added a new exception to detect the case where two
subclasses are being loaded using with_polymorphic(),
each subclass contains a relationship attribute of the same
name, and eager loading is being applied to one or both.
This is an ongoing bug which can't be immediately fixed,
so since the results are usually wrong in any case it raises an
exception for now.   0.7 has the same issue, so an exception
raise here probably means the code was returning the wrong
data in 0.7.  [ticket:2614]