This chage more closely matches the behavior of `get` at runtime. Users can pass whatever they want in to the default
parameter and it will be returned if the key is absent. Additionally, `get` should return an `Optional` if called with
only one parameter.
```python
z = {'a': 22}
reveal_type(z.get('b'))
reveal_type(z.get('b', 22))
reveal_type(z.get('b', 'hello'))
```
Before:
```shell
test_get_default.py:2: error: Revealed type is 'builtins.int*'
test_get_default.py:3: error: Revealed type is 'builtins.int*'
test_get_default.py:4: error: Revealed type is 'builtins.int*'
test_get_default.py:4: error: Argument 2 to "get" of "dict" has incompatible type "str"; expected "int"
```
After:
```shell
test_get_default.py:2: error: Revealed type is 'Union[builtins.int*, builtins.None]'
test_get_default.py:3: error: Revealed type is 'builtins.int'
test_get_default.py:4: error: Revealed type is 'Union[builtins.int, builtins.str*]'
```
