Fix primary key behaviour in bulk_update

Fix primary key behaviour in bulk_update

Suppose you have a model class with a primary key.

Base = declarative_base()
class User(Base):
  id = Column(BigInteger, primary_key=True)
  name = Column(String)

Previously, running
`bulk_update_mappings(User, {'id': 1, 'name': 'hello'})`
would emit the following:

```UPDATE users SET id=1, name='hello' WHERE id=1```

This is contrary to the stated behaviour, where primary keys are omitted
from the SET clause. Furthermore, this behaviour is harmful, as it
can cause the db engine to lock over-aggresively (at least in Postgres).

With this change, the emitted SQL is:

```UPDATE users SET name='hello' WHERE id=1```