BUG: AttributeError when using dict downcast in DataFrame.fillna
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

```python
import pandas as pd
import numpy as np

X = pd.DataFrame(
    {
        "col1": [1, np.nan]
    }
)

X.fillna({"col1": 2}, downcast={"col1": np.int64})
```

#### Problem description

[The docs](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.fillna.html) state that supplying `downcast` as a dict of `item -> dtype` is acceptable here. I would have thought that this would fill NAs in `"col1"` with value `2` and then downcasted `"col1"` to dtype `int64`. Instead an error is raised.

```
AttributeError: 'dict' object has no attribute 'type'
```

#### Expected Output

```
   col1
0     1
1     2
```

#### Output of ``pd.show_versions()``

<details>

```
INSTALLED VERSIONS
------------------
commit           : f2c8480af2f25efdbd803218b9d87980f416563e
python           : 3.7.4.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Mon Aug 31 22:12:52 PDT 2020; root:xnu-6153.141.2~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.3
numpy            : 1.20.1
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 19.0.3
setuptools       : 40.8.0
Cython           : 0.29.22
pytest           : 6.2.2
hypothesis       : None
sphinx           : 3.5.3
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.3
IPython          : 7.21.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.4
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.1
sqlalchemy       : None
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.53.0

```

</details>
