Fix subject matching with multiple class subjects

I came across a little bug when creating a rule with several subjects, here's a snippet to illustrate it:

```python
from bouncer import authorization_method, can
from bouncer.constants import *


class A(object):
    pass


class B(object):
    pass


@authorization_method
def authorize(user, they):
    they.can(READ, (A, B))

print(can(None, READ, A))  # True
print(can(None, READ, B))  # True

a = A()
b = B()

print(can(None, READ, a))  # True
print(can(None, READ, b))  # False, should be True

```

This PR fixes the problem.