Future can't be passed to actor method

**What happened**:

Future is not materialized in an actor method by Dask machinery.

**What you expected to happen**:

Future is able to be materialized in an actor method by Dask machinery how it is done to submit ordinary functions.

**Minimal Complete Verifiable Example**:

```python
from distributed.client import Client

client = Client()

class Foo:
def __init__(self):
self._accumulator = 0
def foo(self, x):
self._accumulator += x

f1 = client.submit(Foo, actor=True)
actor = f1.result()
f2 = client.scatter(7)
f3 = actor.foo(f2)
TypeError: unsupported operand type(s) for +=: 'int' and 'Future'
```

**Anything else we need to know?**:

Related to #4488.

**Environment**:

- Dask version: 2.19.0
- Python version: Python 3.8.8
- Operating System: Ubuntu 20.04.1 LTS
- Install method (conda, pip, source): pip