ensure unhashable types work the same in 1.0 as in 0.9 and former

Before SA 1.0 I could have denormalized schemas where foreign keys pointed to unhashable types (let's say HSTORE). This was very common in same cases for me as I had for example denormalized translation columns using HSTORE as the underlying data type. Then foreign keys with onupdate='CASCADE' where used for automatic real-time denormalization when data changed. I created a simplified test case:

import sqlalchemy as sa
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.dialects.postgresql import HSTORE


engine = sa.create_engine(
    'postgres://postgres@localhost/sqlalchemy_utils_test'
)
Base = declarative_base()
Session = sessionmaker(bind=engine)
session = Session()

session.execute('CREATE EXTENSION IF NOT EXISTS hstore')
session.commit()


class Category(Base):
    __tablename__ = 'category'
    id = sa.Column(sa.Integer, primary_key=True)
    data = sa.Column(HSTORE)
    __table_args__ = (
        sa.Index(
            'some_index',
            data,
            id,
            unique=True
        ),
    )


class Article(Base):
    __tablename__ = 'article'
    id = sa.Column(sa.Integer, primary_key=True)

    name = sa.Column(sa.String)
    category_id = sa.Column(sa.Integer)
    category_data = sa.Column(HSTORE)
    category = sa.orm.relationship(Category)

    __table_args__ = (
        sa.ForeignKeyConstraint(
            [category_data, category_id],
            ['category.data', 'category.id'],
            onupdate='CASCADE'
        ),
    )


Base.metadata.create_all(bind=session.bind)


article = Article(name='Some article', category=Category(data={'1': '2'}))
session.add(article)
session.commit()
category = Category(data={'2': '2'})
session.commit()
article.category = category
session.commit()


Base.metadata.drop_all(bind=session.bind)

throws Exception

...
    elif orm_util._never_set.intersection(params.values()):
TypeError: unhashable type: 'dict'

The problem is even deeper than this as the following code block illustrates:

import sqlalchemy as sa
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.dialects.postgresql import HSTORE


engine = sa.create_engine(
    'postgres://postgres@localhost/sqlalchemy_utils_test'
)
Base = declarative_base()
Session = sessionmaker(bind=engine)
session = Session()

session.execute('CREATE EXTENSION IF NOT EXISTS hstore')
session.commit()


class Article(Base):
    __tablename__ = 'article'
    id = sa.Column(HSTORE, primary_key=True)


Base.metadata.create_all(bind=session.bind)


article = Article(id={'1': '2'})
session.add(article)
session.commit()


Base.metadata.drop_all(bind=session.bind)

Which throws exception:

Traceback (most recent call last):
  File "sa_test.py", line 42, in <module>
    session.commit()
  File ".../sqlalchemy/orm/session.py", line 790, in commit
    self.transaction.commit()
  File ".../sqlalchemy/orm/session.py", line 392, in commit
    self._prepare_impl()
  File ".../sqlalchemy/orm/session.py", line 372, in _prepare_impl
    self.session.flush()
  File ".../sqlalchemy/orm/session.py", line 2004, in flush
    self._flush(objects)
  File ".../sqlalchemy/orm/session.py", line 2122, in _flush
    transaction.rollback(_capture_exception=True)
  File ".../sqlalchemy/util/langhelpers.py", line 60, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File ".../sqlalchemy/util/compat.py", line 182, in reraise
    raise value
  File ".../sqlalchemy/orm/session.py", line 2086, in _flush
    flush_context.execute()
  File ".../sqlalchemy/orm/unitofwork.py", line 373, in execute
    rec.execute(self)
  File ".../sqlalchemy/orm/unitofwork.py", line 532, in execute
    uow
  File ".../sqlalchemy/orm/persistence.py", line 149, in save_obj
    base_mapper, states, uowtransaction
  File ".../sqlalchemy/orm/persistence.py", line 292, in _organize_states_for_save
    instance_key in uowtransaction.session.identity_map:
  File ".../sqlalchemy/orm/identity.py", line 96, in __contains__
    if key in self._dict:
TypeError: unhashable type: 'dict'

The second issue is not critical for me but the first is. I have many projects using HSTORE based translations and denormalization.

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