SQLA defaults set with text() throw an exception.

Code changed to allow for objects such as:
class User(Base):
    __tablename__ = 'user'
    id = Column(BigInteger(unsigned=True), default=text("uuid_short()"),
                primary_key=True, autoincrement=False)

default can't be interpretted before calling SQLA's backend so Colander's defaults are left blank and field is set to not be required (since it being left empty will call the default)