cleanly upgrade the type of bindparam() with a null type

[Mike Bayer]

        bp = bindparam("somevalue")
        stmt = test_table.update().values(avalue=bp)
        compiled = stmt.compile()
        eq_(bp.type._type_affinity, types.NullType)
        eq_(compiled.binds['somevalue']('somevalue').type._type_affinity, MyCustomType)

# additionally:

        bp = bindparam("foo")
        expr = column("foo", MyFoobarType) + bp
        assert bp.type._type_affinity is types.NullType
        assert expr.right.type._type_affinity is MyFoobarType

[Mike Bayer]

we're looking here to _clone the bindparam() so that we don't need to mutate in place.

[Mike Bayer]

9caa92b96fef0b3e6d9d280407bc6a09

[Mike Bayer]

Removing milestone: 0.9.0 (automated comment)