TypeError: unhashable type

Hi,

when I pass my Falcon resource to apispec I got this error:

````
loginResource = LoginResource(user_service)
deviceResource = DeviceResource()
api.add_route('/api/login', loginResource)
api.add_route('/api/devices', deviceResource)

# Create an APISpec
from apispec import APISpec
from apispec.ext.marshmallow import MarshmallowPlugin
from falcon_apispec import FalconPlugin

spec = APISpec(
title='Swagger Backend API',
version='1.0.0',
openapi_version='2.0',
plugins=[
FalconPlugin(api),
MarshmallowPlugin(),
],
)

# pass created resource into `path` for APISpec
spec.path(resource=deviceResource)
````

````
$ python -u test.py
Traceback (most recent call last):
File "test.py", line 214, in
spec.path(resource=deviceResource)
File "C:\Work\Analytics\git\test\falcon-test\.venv\lib\site-packages\apispec\core.py", line 347, in path
path=path, operations=operations, parameters=parameters, **kwargs
File "C:\Work\Analytics\git\test\falcon-test\.venv\lib\site-packages\falcon_apispec\falcon_plugin.py", line 30, in path_helper
resource_uri_mapping = self._generate_resource_uri_mapping(self._app)
File "C:\Work\Analytics\git\test\falcon-test\.venv\lib\site-packages\falcon_apispec\falcon_plugin.py", line 23, in _generate_resource_uri_mapping
mapping[resource] = uri
TypeError: unhashable type: 'LoginResource'
````

which is not on the resource I have added...
Any idea ?

Thanks!